问题补充：
.
--- What can I do for?
--- I’d like to buy a gift for my sister’s birthday, one at a proper price but of great use and it’d better be surprise.
A. /; a
B. the; a
C. a; /
D. /; the
答案：
(答案→)A. /; a
B. the; a
C. a; /
D. /; the
解析：of great use=useful第一空不填， surprise 抽象名词具体话，一件惊讶的礼物用a
