单选题 In the author's opinion, looking up all the words you don't know when you are reading in English is______.

【正确答案】 C
【答案解析】事实细节题。根据第一段第二句:Looking these up in a dictionary can be very time-consuming and frustrating(查字典可能很费时间而且令人心烦)。所以选项C是正确答案。