一个正方体的表面积与它的底面积的比是______．
设正方体的棱长是a，则：
6a2：a2=6：1；
故答案为：6：1．
从1,2,3,4,5,6这6个数字中，任取2个数字相加，其和为偶数的概率是________．
It was not until 1911, _______ the first vitamin was identified, _______ people realized the importance of it．
A．when; which
B．when; that
C．that; which
D．that; that
