—Why are there so many policemen in the park?
—They are trying ________ a missing girl.
A:search
B:to search
C:search for
D:to search for
下列海产品中,属于我国四大海产品的是( )
①大黄鱼 ②海带 ③带鱼 ④乌贼 ⑤小黄鱼 ⑥紫菜.
CH4、X、H2O、HF四种化合物是按它们的相对分子质量和化合价而有规律地排列的,其中X应该是( )