Review the proof of cos(A - B) = cosAcosB + sinAsinB.

Step 1: √(cosa−cosb)²(sina−sinb)²
Step 2: (cosa−cosb)²(sina−sinb)²=(cos(a−b)−1)²(sin(a−b)−0)²
Step 3: (cos²a−2cosacosb+cos²b)(sin²a−2sinasinb+sin²b)
Step 4: cos²a−2cosacosb+cos²b=cos²(a−b)
Step 5: sin²a−2sinasinb+sin²b=sin²(a−b)
Step 6: cos²(a−b)⋅sin²(a−b)
Step 7: cos²(a−b)⋅sin²(a−b)=cosacosbsinasinb
Which of the following complete step 4 of the proof?

a.1 and 1
b.2 and 1
c.(cosAcosB)²(sinAsinB)² and (cos²(A – B))((sin²(A – B))
d.(cos²A + sin²A)(cos²B + sin²B) and (cos²(A – B))(sin²(A – B))