Four people live in a dormitory. One night, the last person to come back forgot to turn off the light. The administrator asked who came back the latest.

A said that when I came back, C hadn't come back.

B said that Ding had fallen asleep when I came back, and so did I.

C said that B was sleeping when I came in.

Ding said I went to sleep when I came back, and I didn't pay attention to anything else.

What they said is true. Do you know who came back last?

It is C.

A means that a is earlier than C.

B means d is earlier than B.

C means that b is earlier than C.

Because what they said is true.

So a, b and d are all earlier than C.

C will be back at the latest.