题目

如图所示，在竖直平面内有一半圆，其直径水平且与另一圆的底部相切于O点，O点恰好是下半圆的圆心，现在有三条光滑轨道AB、CD、EF，它们的上下端分别位于上下两圆的圆周上，三轨道都经过切点O，轨道与竖直线的夹角关系为α＞β＞θ，现在让一物块先后从三轨道顶端由静止下滑至底端，则物块在每一条倾斜轨道上滑动时所经历的时间关系为（ ）AtAB＞tCD＞tEFB．tAB＜tCD＜tEFC．tAB=tCD=tEFD．无法确定根据括号中所给的汉语提示写出单词，使句子意思完整。（5分）1.He made a__________( 决定)to accept the challenging task..2.I hurt my_______(膝盖),and it’s painful when I walk.3.It’s__________(必需的) to drink eight glasses of water every day.4.Mo Yan is a famous________(作家).5.He was so_______(瞌睡的) that he could hardly keep his eyes open.