试述筒壳、扁壳结构的组成和特点是什么？
参考答案：
壳身、边梁和横隔。双向轴力和受剪，下部环向受拉，故边梁同样有防裂和平衡作用，横隔起抗剪作用。
不等式x2-2x-3≤0的解集是 [ ]
A.{x|x≤-1}
B.{x|x≥3}
C.{x|x≤-1，或x≥3}
D.{x|-1≤x≤3}
She looks _____ because she has a _____ vacation.[ ]
A. relaxed, relaxed
B. relaxing, relaxing
C. relaxing, relaxed
D. relaxed, relaxing
