Boy: You 3 ______ start revising now. You 4 ______ wait.

Explanation:

The boy is responding to the girl's request for advice about her exam. He suggests starting to revise. The first blank, 'You 3 ______ start revising now', requires a modal verb indicating a suggestion or strong recommendation. 'Should' is appropriate here. The second blank, 'You 4 ______ wait', implies that waiting is not the best course of action. Therefore, 'shouldn't' or 'can't' would fit, with 'shouldn't' being a direct contrast to the recommendation of starting revision, and 'can't' implying impossibility or strong disapproval of waiting.

Answer:

Boy: You 3 should start revising now. You 4 shouldn't wait.