My brother usually ___ his classroom after school. But now he ___ basketball.

A．cleans; plays

B．is cleaning; plays

C．cleans; is playing

D．is cleaning; is playing

在能见度不良时的安全航速是指：（）

A.以能维持舵效的速度航行

B.能在能见距离一半的距离内把船停住的速度

C.能采取适当而有效的避碰行动，并能在适合当时环境和情况的距离以内把船停住的速度

D.当听到雾号显似在本船正横前时，能够维持舵效的最低速度