高积云可与（）相互演变。
A.高层云
B.卷积云
C.层积云
参考答案：A, B, C
He spent as much time as he could ______ his spoken English. [ ]
A. improve
B. improving
C. to improve
D. improved
侧梁、端梁、枕梁、横梁、斜撑梁翼板横裂纹长度大于翼板宽的50%但未延及腹板时补（）补强板。
