Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Used To:  Used to has no Present tense. Used is following by the full infinitive to.

1. To express a discontinued habit or a past situation which is no more in the present. e.g.  He used to drink daily. ( Now he does not drink )

In the question : -

1. I ___________  think that it would be a great idea to live there.

In this sentence , used to is the correct modal that can be placed in the blank part as it is used to say that something happened continuously or frequently during a period in the past .

2. We ___________ go sailing on the lake in summer.

In this sentence , used to is the correct modal that can be placed in the blank part as it is used to say that something happened continuously or frequently during a period in the past .

Thus , the Correct Option is C , which is 1. Used to 2. Used to .

Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Used To:  Used to has no Present tense. Used is following by the full infinitive to.

1. To express a discontinued habit or a past situation which is no more in the present. e.g.  He used to drink daily. ( Now he does not drink )

In the question : -

1. I was _________ driving through crowded streets .

In this sentence , used to is the correct modal that can be placed in the blank part as it is used as an adjective   (be used to)  It is equal to 'accustomed to' .

2. She  ___________ work in office before marriage. 

In this sentence , used to is the correct modal that can be placed in the blank part as it is used to denote Discontinued Action  .

Thus , the Correct Option is D , which is 1. Used to 2. Used to .

Modals: They are verbs which are used before the main verb to express ability, possibility, obligation, permission, suggestion, request, certainty or necessity.They indicate the attitude, intention and the mood of the speaker.

• They cannot be used alone in a sentence.
• In case of Questions they are used alone.
• They are not affected by the change in person i.e. first, second or third form.
• Present Participle or Past Participle form of a verb is never used with a modal.

Use of Used To:  Used to has no Present tense. Used is following by the full infinitive to.

1. To express a discontinued habit or a past situation which is no more in the present. e.g.  He used to drink daily. ( Now he does not drink )

In the question : -

1. She ___________ work for a large insurance company.

In this sentence , used to is the correct modal that can be placed in the blank part as it is used to show something happened continuously or frequently during a period in the past .

2. You _________  see a lot of her, didn't you?

In this sentence , used to is the correct modal that can be placed in the blank part as it is used to show something happened continuously or frequently during a period in the past .

3. I didn't ____________  like him much when we were at school.

In this sentence , use to is the correct modal that can be placed in the blank part as it is used to show something happened continuously or frequently during a period in the past . Use is used in affirmative sentences .

Thus , the Correct Option is D , which is 1. Used to 2. Used to 3. Use to .