抽样中最基本的因素是（）。
参考答案：确定目标人群或总体
We go out for a walk every evening. My grandfather goes with us _____ . [ ]
A. well
B. as well
C. as well as
D. very well
设p、r都是q的充分条件，s是q的充分必要条件，t是s的必要条件，t是r的充分条件，那么p是t的______条件，r是t的______条件．
