题目

--The lake is said to be dry. Is that true?    --It_______ .Look, some kids are swimming in it.     A. must be true     B. can't be true     C. may not be true       D. is true 答案：B下列关于σ键和π键的说法正确的是（　　） A、单键既有σ键又有π键B、所有的π键都容易打开C、σ键可沿键轴自由旋转而不影响键的强度D、π键比σ键重叠程度大，形成的共价键强