_________ caused the accident is still a mystery.
| A．How | B．What | C．That | D．Where |
. Why shouldn’t I buy a new coat — I haven’t bought _____ for five years.
| A．it | B．that | C．one | D．which |
. He made _____ known to his friends that he didn’t want to enter politics.
| A．that | B．it | C．himself | D．him |
. It used to be thought _____ the Earth was flat.
| A．as | B．when | C．since | D．that |
. In the west, people make _______ a rule to send Christmas present to their relatives and friends.
| A．this | B．that | C．it | D．the following |
. _______ is well known, _______ Hong Kong returned to China on July st, .
| A．It, that | B．As, / | C．As, as | D．It, which |