The weather in spring is like --- baby's face.It changes qui

The weather in spring is like ___ baby's face.It changes quickly.A、the B、a C、an D、/
请说明答案的原因,谢谢!还有,这是谚语吗?
我选了D，觉得语感是这样，能说说为什么不选D的原因吗？