已知等腰三角形的一边长是3厘米，另一边长是7厘米，这个等腰三角形的周长是（ ）厘米。
17
The split between humans and living apes is thought by some scholars ____ 15 to 20 million years ago.[ ]
A. to occur
B. occurring
C. to have occurred
D. having occurred
如图所示，在长度一定的细线下方系一重量为G的小球，线的另一端固定，使悬线与竖直方向的夹角为θ=60°时无速释放小球．则小球摆回到最低点P时，细线所受力的大小是______．
and living apes is thought by some scholars ____ 15 to 20 million years ago.[ ]