A、On Saturdays. B、On Sundays. C、On Thursdays. D、Every day. A根据选项，推测对话的内容与时间相关。对话中“I always clean it on Saturdays．”给出了答案。因此答案是A。

W: It’s almost five o’clock and Mark’s not here yet.
M: If he doesn’t hurry, he’ll miss the train.
Q: What are they worried about?

M: How often do you clean your apartment?
W: About once a week. I always clean it on Saturdays. But I clean my kitchen every day and I sweep the floor on Sundays, Thursdays and sometimes Saturdays, too.
Q: When does the woman clean her apartment?