用显微镜观察洋葱根尖有丝分裂实验时，先用低倍镜观察，后用高倍镜观察，所观察到的，后者比前者[ ]

A．物象较小，视野较亮

B．物象较大，视野较亮

C．物象较小，视野较暗

D．物象较大，视野较暗

A carbon dioxide fire extinguisher should be recharged（）.

A.at least annually

B.whenever it is below its required weight

C.only if the extinguisher has been used

D.before every safety inspection