There is a lot of love in today's world and I'm convinced that this is still the most powerful _____ in the world. [ ]

A. power

B. energy

C. force

D. strength

He _____ in the classroom just now. he _____ be there now. [ ]

A. heard to sing, may

B. was heard sing, must

C. heard sing, must

D. was heard to sing, may