表兄妹之间不能结婚,从法律上讲是因为表兄妹( )
A:属于直系血亲
B:属于三代以内的旁系血亲
C:遗传基因十分接近
D:不能优生
将左边和右边的句子连成对话。
①Do you like giraffes?____ A. It's a snake.
②What's that?____ B. I hear tigers.
③What colour are the pandas?____ C. No, I don't.
④What do you hear? ____ D. They're black and white.
观察图形,回答问题:
如图按上面的方法继续下去,猜测第n个图形中有____个三角形(用n的代数式表示结论).