If we assume that students study only for 3 years, then the answer
should be D

3rd year: 40%
2nd year: x
1st year: y

y + 40 = 70 implies y = 30
If y = 30, x = 30 as 40 + x + y = 100

So, y / (x+y) = 30 / 60 = 1/2