1 . ----There are some pens in his house but I can never find one.
----Keep looking. ______ is sure to appear.

A．One

B．It

C．That

D．This

2 . This is a simple idea, but        that is hard to put into practice.

A．one

B．it

C．that

D．those

3 . ______the students didn’t hear what the teacher said. Some of them were absent-minded .

A．None

B．Not all

C．All

D．Neither