1 . The greatest mystery is ______ caused the Maya to abandon most of their great cities.

A．that

B．what

C．which

D．who

3 . Language reflects the creativity of the human race. That’s ______ when the stars are out, they are visible, but when the lights are out, they are invisible.

A．why

B．because

C．how

D．if

4 . The last time we met each other was ________ I was on a business trip in her city.

A．where

B．how

C．when

D．why

5 . He did not see the film last night. That is _______ he had to help his little sister with her homework.

A．why

B．because

C．that

D．how

6 . See the flags on top of the building? That was ______ we did this morning.

A．when

B．which

C．where

D．what

7 . It is generally believed that communication skills are becoming ________ it takes to be a good doctor.

A．that	B．what
C．whether	D．how