The following four statements are parts of a proof that the sum of two odd integers is even:
P. Substitute x=2k+1 and y=2ℓ+1, where k and ℓ are integers.
Q. x+y=2(k+ℓ+1).
R. 2(k+ℓ+1) is divisible by 2 and hence is even.
S. Therefore, the sum of two odd integers is even.
Which order arranges these statements into a logically correct paragraph proof?
a) Q → P → R → S
b) P → R → Q → S
c) P → Q → R → S
d) P → Q → S → R