Answer the following question based on the information given below.
f1(x) = x 0 ≤ x ≤ 1
= 1 x ≥ 1
= 0 otherwise
f2(x) = f1(–x) for all x
f3(x) = –f2(x) for all x
f4(x) = f3(–x) for all x
How many of the following products are necessarily zero for every x
f1(x)f2(x), f2(x)f3(x), f2(x)f4(x)?