Your friend needs to gain weight to wrestle in a heavier weight class. His goal is to gain 2 pounds per week. After 3 weeks, his weight is up to 158 pounds. If the rate of weight gain stays the same, which equation shows his weight at the end of each week?

A: y−3=2(x−158)
B: y-158=2(x+3)
C: y−158=3(x−2)
D: y−158=2(x−3)