51. in class, in the class
in class在课上，in the class在班级里 He is the best student in the class.
52. on fire, on the fire
on fire着火,on the fire在火上 Put the food on the fire. The house is on fire.
53. out of question, out of the question
out of question毫无疑问的，out of the question不可能的
