After several years of observation, the research group ______ the conclusion that language acquisition is

faster when students ______ to the situation where it is spoken.[ ]

A. has come to; expose              

B. have arrived at; are exposed

C. has concluded; are exposed        

D. have reached; expose

下列说法正确的是（）

A、船舶本身在诉讼中可以作为被告而被原告起诉

B、船舶是动产，适用动产的法律

C、根据我国《船舶入级规则》规定，船级证书的有效期为3年

D、船舶国籍证书的有效期为4年