— Why is the classroom so dirty?

— Sorry, sir. It             yesterday. We forgot to do it. [ ]

A．doesn't clean

B．didn't clean

C．isn't cleaned

D．wasn't cleaned

脑栓塞栓子主要来源是

A．动脉粥样硬化斑块脱落

B．心源性栓塞

C．肺静脉血栓或血凝块

D．血液的高凝状态

E．脂肪栓塞