— Hello, is Billy in? 

— Sorry, he isn't in, ____[ ]

A. Billy, here is a call for you.

B. can you take a message?

C. go ahead, I'll give him a message.

D. Billy, get the call.

关于酶的化学修饰的特点，错误的是

A．最常见的化学修饰是磷酸化和去磷酸化
B．是与酶蛋白水解无关的可逆共价修饰
C．不改变酶的构象
D．酶以无活性和有活性两种形式存在