俄罗斯人到中国最有可能采购的商品是( )
A:铁矿
B:煤炭
C:服装
D:石油
— It's dangerous to swim in this river.
—Yes, you are right. The government has ______ people not to swim in it.
解释下列词语
①坚硬
②棱角
在下面正方体的任意一个面上,画一个尽可能大的圆.再求出这个圆的周长.