51. in class, in the class　

in class 在课上；
in the class 在班级里，例：He is the best student in the class.

52. on fire, on the fire　

on fire 着火，例：The house is on fire.
on the fire 在火上，例：Put the food on the 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。

54. a second, the second　

a second 又一，再一；
the second 第……，例：He won the second prize.

55. by day, by the day　

by day 白天；
by the day 按天计算，例：The workers are paid by the day.