语法与词汇 The European Union countries were once worried that they would not have _____ supplies ofpetroleum.

【正确答案】 D

【答案解析】句意：欧盟成员国曾一度担心他们将不会有充足的石油供应。sufficient充足的。proficient熟练的。efficient有效的。potential潜在的。因此，本题的正确答案为D。