放射性物质进入人体内的途径主要有食入、吸入和（）。
参考答案：皮肤吸收
51÷17=3，17和3都是51的因数。[ ]
It is good news to us that we are invited to such a great concert, _____?
[ ]
A. is it
B. isn't it
C. are we
D. aren't we
