1. Whether to do each of five projects is represented by binary variables y1, y2, y3, y4, and y5. At most, 2 projects can be done. The constraint that implements this is

a. y1 + y2 + y3 + y4 + y5 <= 5

b. y1 + y2 + y3 + y4 + y5 = 2

c. y1=1, y2=1, y3=0, y4=0, y5=0

d. y1 + y2 + y3 + y4 + y5 <= 2

2. Project 1 (binary variable y1) can only be done if project 2 (binary variable y2) is also done. The constraint that implements this is

a.y1 = y2

b. y1 <= y2

c.y1 >= y2

d.y2 = IF(y1=1, 1, 0)