an open book
昭然若揭的事
A:What!Your golden watch is lost?
A: 什么?你的金表丟了?
B: Yeah.I just can't find it anywhere.
B:是啊,我哪兒也找不到。
A: It's an open book that it was stolen by the man in black.
A: 很明顯是那個穿黑衣服的人偷的。
B: Let's get it back.
B:讓我們去找回來。