应当用 B. between.openbetween 10a.m. and 3:30p.m表示时段,10点到下午3点开着.
at 是 时间点, 精确的时刻.at 10am 就是 10点这个时刻.
除非是 at midnight on Christmas Eve.这个 at 是 night 的要求.
具体钟点,不能 at 10am and 3:30pm.

商店 open 常用
open 10am - 3:30pm,
open 10am to 3:30pm,
open 10am through 3:30pm,
open from 10am to 3:30pm,

是否答案有误.