【题目】To improve learning efficiency, students must spend more time focusing on the skills__________ they are weak in.
A.whatB.whereC.whenD.which
在线课程【答案】D
【解析】
考查定语从句关系词。分析句子结构可知，句子“they are weak in”是定语从句，修饰先行词the skills，先行词指事物，在从句中充当介词in后面的宾语成分，需用关系词which。故选D。