—Look at the boy running on the ground. Is it David?       

 —It _____be him. I saw him go to the teacher's office just now.        [ ]

A. might    

B. could    

C. can't    

D. mustn't

（）是来自地球深处的可再生热能。它起源于地球的熔融岩浆和放射性物质的衰变。