患甲状腺肿大(俗称大脖子)的病人体内缺少的元素是 [ ]

A.碘   

B.钙  

C.铁   

D.锌

Don't use water to put out the fire if oil catches fire. As is known, oil ________ float on water.[ ]

A．shall        

B．must

C．may  

D．will