Deduction and Posibility

Have a drink of water. You -------- might / must------ be very thirsty after the match

I don´t know what is in the box. It -----could / can --------- be a present

Harry failed two exams. He --------couldn´t / can´t be -------- very happy

The doctor is not very sure. She says it --------might / must -------- be serious

I am not sure but I thing this gold -------- may not / can´t be -------- real

Reading in bed at night --------can / must --------help you fall asleep quicker