an open book
昭然若揭的事
A：What!Your golden watch is lost?
A: 什么?你的金表丢了?
B: Yeah.I just can't find it anywhere.
B：是啊，我哪儿也找不到。
A: It's an open book that it was stolen by the man in black.
A: 很明显是那个穿黑衣服的人偷的。
B: Let's get it back.
B：让我们去找回来。