Soon after getting off his horse, the man appeared at the second storey windows,   __  he could see nothing but trees.

A．from which

B．from where

C．in where

D．which

用N_A表示阿伏加德罗常数的数值，下列说法中正确的是（    ）

A．常温常压下，氧气和臭氧的混合物16g中含有N_A个氧原子

B．常温下，5.6g铁与足量的盐酸反应，失去的电子数为0.3N_A

C．盛有CO₂的密闭容器中含有N_A个氧原子，则CO₂的物质的量不一定是0.5mol

D．1mol·L^－1NaCl溶液含有N_A个Na⁺离子