A:unle
B:while
C:although
D:once
下面是对双子叶草本植物种群密度的取样调查,操作正确的是( )
①确定调查对象 ②选取样方 ③计数 ④计算种群密度.
My headache _________ me. I thought it was going away, but now it's getting worse.