单选题 ______ fairly recently, ______ solved, at least partially.
A. Until… that this problem was
B. It was until… that this problem was
C. Not until… was this problem
D. It was not until… when this problem was

【正确答案】 C

【答案解析】[解析] until可以与not连用．表示“直到……(才)”。如果not until放到句首，其后面部分需部分倒装。根据这一点，可得出C为正确选项。