以下对酵母菌的叙述，正确的是（　　）

A．酵母菌属于单细胞真菌

B．酵母菌没有叶绿体，营寄生生活

C．酵母菌没有真正的细胞核

D．酵母菌的细胞结构与动物细胞相似

—We're doing a lot to protect our environment, but it is not good enough.

—So we should try ______ to look after it. [ ]

A. hard

B. harder

C. hardest

D. hardly