单选题 The solution cannot be determined _____the problem is identified.

A、 (A) if
B、 (B) when
C、 (C) until
D、 (D) which

【正确答案】 C

【答案解析】Until indicates a time or an action that another action depends upon. Choice (A) is conditional and is not logical in the sentence. Choice (B) indicates a time relationship but is not logical in the sentence. Choice (D) is a relative pronoun.