Most birds find it safe to sleep in the trees, but          they have eggs or young chicks, they don't use a nest.[ ]

A. why

B. how

C. unless

D. where

茗茗向一定溶质质量分数的氯化铵溶液中加入12.5g氯化铵，完全溶解后，得到溶质质量分数为30%的氯化铵溶液100g，则原氯化铵溶液溶质质量分数约为（　　）

A．10%

B．11%

C．20%

D．22%