The greatest common divisor of a and b is 21, and the least common multiple of a and b is 126, where a and b are positive integers, what is the sum of a and b? (A)105 (B)147 (C)150 (D)105 or 147 (E)105 or 150 按理说这应该是很简单的提醒吧,但我碰到公约数公倍这类题型就是不知道怎么做,过两天就考了,请各位帮帮忙~~有什么惯用的解题思路波