Circle ‘that’ if it can be left out in the following sentences

1. The scientist introduced the idea, but it was her assistants that completed the work.

(that can't be left out)

Emphasized information: her assistants completed the work.

2. Don’t thank me. It was everyone’s efforts that we should be thankful for.

(that can't be left out)

Emphasized information: everyone's efforts to be thankful for.

3. It was last weekend that he came to see us for the first time.

(that can't be left out)

Emphasized information: he came to see us for the first time last weekend.

4. It is to gain experience that we want to take part in this volunteer work.

(that can't be left out)

Emphasized information: we want to take part in this volunteer work to gain experience.

5. All of my teammates are excellent students, but it is Mary that I admire the most.

(that can be left out)

Emphasized information: Mary is the one I admire the most among all of my excellent teammates who are students.