21.The books in David’s schoolbag felt like bricks as he ran down the street. What he wanted to do was to play basketball with Eric, 21 his mother told him he would have to return his sister’s books to the library first.He had 22 set foot in a library and he wasn’t about to do so today. He would just 23 the books in the outside return box. But there was a 24 ； it was locked.He went into the building, only a few minutes 25 closing time. He put the books into the return box. And after a brief 26 in the toilet, he would be on his way to the playground to 27 Eric.David stepped out of the toilet and stopped in 28 — the library lights were off. The place was 29 . The doors had been shut. They 30 be opened from the inside, he was trapped（被困） — in a library!He tried to 31 a telephone call, but was unable to 32 . What’s worse, the pay phones were on the outside of the building. 33 the sun began to set, he searched for a light and found it.34 he could see. David wrote on a piece of paper： “ 35 ! I’m TRAPPED inside!” and stuck it to the glass door. 36 , someone passing by would see it.He was surprised to discover that this place was not so unpleasant, 37 . Rows and rows of shelves held books, videos and music. He saw a book about Michael Jordan and took it off the shelf. He settled into a chair and started to 38 .He knew he had to 39 , but now, that didn’t seem to be such a 40 thing.
(A)but
(B)because
(C)or
(D)since