He didn't show up at the party yesterday. He( )in the lab.( )
A.must be busy working B.must be busy to work C.must have been busy working D.would have been busy working正确答案C
He didn't show up at the party yesterday. He( )in the lab.( )
A.must be busy working B.must be busy to work C.must have been busy working D.would have been busy working正确答案C