Great changes ___ in our city since 1977.
| A．take place | B．have taken place | C．took place | D．are taken place |
It is uncertain ________ side effect the medicine will bring about, although about two thousand patients have taken it.(2010·浙江)
| A．that | B．what | C．how | D．whether |
Part of the reason Charles Dickens loved his own novel, David Copperfield, was ________ it was rather closely modeled on his own life.(2010·北京)
| A．what | B．that | C．why | D．whether |
He didn't go by bike with me.That's ________ there was something wrong with his bike.
| A．because | B．why | C．how | D．when |
They________have arrived at lunchtime but their flight was delayed.(2011·全国 )
| A．will | B．can | C．must | D．should |
—Must I turn off the gas after cooking?
—Of course.You can never be ________careful with that.
| A．enough | B．too | C．so | D．very |