From the looks of it I believe it is B, about 74. It is this because when you find the y value of 400, continue on the 400 line until you find what the value of x would be. In this case, it appears to be slightly before 75, (we know this because it is between 70 and 80 do the middle would be ten). If it were about 72 it would intercept closer to the 70 mark.
