A vessel is constructed with a steel hull and an aluminum superstructure. Which statement is TRUE?（）

A.The aluminum will provide greater resistance to the spread of fire by conduction

B.The aluminum structure is usually attached to a steel coaming by a method that insulates the two metals

C.If the superstructure is stressed，an aluminum structure requires additional expansion joints to prevent fracture

D.The steel at the area of the aluminum-to-steel connection must be closely checked for galvanic corrosion

下列特征中哪个不是经典超级单体风暴的典型特征：（）

A、钩状回波；

B、有界弱回波区；

C、中层径向辐合；

D、中气旋