补全对话。    

A: Hello, may I speak 1_____ Lili?

B: Sorry, Lili isn't 2_____. Is that you, Beibei?

A: Yes, Beibei is speaking. Who's 3_____, please?

B: 4_____ is Lili's mother. Lili told me you would come to help her 5_____ her English.

A: Yes, but I can't do that this evening.

B: 6_____ not?

A: My aunt will arrive this evening. I'll have to 7_____ her at the airport.

B: I'm 8_____ to hear that.

A: Can you pass a message 9_____ Lili?

B: Yes, of 10_____. 

A: Thank you. Goodbye!

B: Bye-bye!

IsNull的含义是( )

A．字段值是否为空

B．字段值为空

C．字段值非空

D．字段值默认