— He is so clever.
A:his; himself
B:his; by himself
C:him; himself
D:him; by himself
用反证法证明命题“一个三角形中至多有一个角是直角”,应先假设这个三角形中( ).
“一身乌黑光亮的羽毛,一对俊俏轻快的翅膀,加上剪刀似的尾巴,这就是活泼机灵的小燕子.”下面是燕子的一组形态结构图: