题目

Kids are ____ their parents’approval, but some parents never seem satisfied with their kids’ behavior. A.concerned about   B. worried about   C.eager for    D. surprised at   答案：C  类比平面内直角三角形的勾股定理，在空间四面体P-ABC中，记底面△ABC的面积为S0，三个侧面的面积分别为S1，S2，S3，若PA，PB，PC两两垂直，则有结论S20=S21+S22+S23S20=S21+S22+S23．