主流河槽处流速大，河滩处流速小。
参考答案：对
There is no doubt ________ China has made a great breakthrough in developing education in the
countryside.[ ]
A. /
B. which
C. that
D. what
建立概念模型一般包括（）个步骤。
A.4
B.5
C.6
D.7
