The difference is ________ one is good conductor of electricity, and the other is not．

A．this

B．that

C．why

D．how

Mary is not ________ she was before．

A．that

B．what

C．who

D．which

________ we shall go for our practice has not been tried．

A．Why

B．If

C．Where

D．What

________ this kind of chemical is better has not been tried．

A．What

B．That

C．Whether

D．Which

________ was that the plane had reached the sound barrier．

A．That had happened

B．That happened

C．Which had happened

D．What happened

The reason for his hesitancy was ________ he did not like to blame anyone without proof．

A．which

B．what

C．that

D．why

He offered me a set of rooms to ________ should find the lost child．

A．whom

B．whomever

C．whoever

D．anyone

The other day my brother drove his car down the street at ________ I thought was a dangerous speed．

A．as

B．which

C．what

D．that

Parents are taught to understand ________ important education is to their children's future．

A．that

B．how

C．such

D．so

We cannot figure out ________ quite a number of insects, birds, and animals are dying out．

A．that

B．as

C．why

D．when