高等植物细胞有丝分裂中，下列直接参与活动的细胞器有：[     ]

A．高尔基体和核糖体

B．高尔基体和中心体

C．叶绿体和核糖体

D．中心体、高尔基体和核糖体

For a floating vessel, the center of buoyancy and the metacenter are in the line of action of the buoyant force（）.

A.only when there is positive stability

B.only when there is negative stability

C.only when there is neutral stability

D.at all times