生物圈中有着多种多样的生态系统，下列不属于生态系统的是 [ ]

A．一块农田

B．一座城市

C．一群正在吃草的羊

D．一条河流

It is uncertain ______ side effects these weight-loss drugs will bring about, although hundreds of thousands of patients have taken it.

A．that

B．whether

C．how

D．what