Subject: mathqa
Question: there are two groups of students in the sixth grade . there are 45 students in group a , and 55 students in group b . if , on a particular day , 20 % of the students in group a forget their homework , and 40 % of the students in group b forget their homework , then what percentage of the sixth graders forgot their homework ?
Option: 
A: 23 %
B: 31 %
C: 36 %
D: 25 %
Answer: B