_____ is certain is ______ the mystery of Stonehenge has never been fully explained.
| A．It, that | B．What, what |
| C．It, what | D．What, that |
I ____ to visit you later that day, but I had to phone and cancel.
| A．come | B．came | C．am coming | D．was coming |
Animals are obviously _____ lower form of life than ______ man.
| A．a; / | B．the; the | C．a; the | D．/; / |
Recycling is one way to protect the environment; reusing is _____.
| A．another | B．the other | C．one another | D．one |
I would have gone to my cousin’s birthday party last night, but I ______ available.
| A．was | B．had not been | C．was not | D．would not be |
______ is often the case in science, there is no simple answer
| A．That | B．As | C．When | D．Where |