Robert Collier once said, “A single idea---the sudden flash of a thought

--- __________be worth a million dollars.”

A．should

B．may

C．must

D．will

吸烟有害健康．吸烟时产生一种能与人体血液中的血红蛋白结合的有毒气体是　 [ ]

A． N₂

B． SO₂

C． CO

D． CO₂