(since/ for)-- how long have you benn here?

题目

(since/ for)-- how long have you benn here?
(since/ for)
-- how long have you benn here?
--__(since/ for) vhalf past three.
选择哪个?为什么?

答案

Since 自从3点半开始
since+时间点 从XX时候开始
for+一段时间 持续了XX时间