Please revise the following sentences by adding commas or semicolons where they are needed. Also, offer your explanation for why you added the commas or semicolons.

7. Throwing his arms into the air, Mike looked absolutely dumbfounded, so he stood up, knocking all of the materials on the floor, and left the building.

Rewritten sentence:

Reason for punctuation:

8. Since the expansion of the Internet research has become much less tedious. Rewritten sentence:

Reason for punctuation:

9. To maintain excellent grades in your classes is an accomplishment. Rewritten sentence:

Reason for punctuation: