从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21.—Where did ________traffic accident happen, sir ?
—It took place at ________crossroad by the bank.
A. a; / B. the; a C. /; a D. a; a
22. Scientists have found no ________of life on Mars so far, though great efforts have been made on the research.
A. symbols B. signs C. marks D. signals
