Let $S$ be the set of nonnegative continuous functions $f$ on $[0,1]$ satisfying
\[ \int_{0}^{1} \sin ^{2}(x) f(x) d x=\int_{0}^{1} \sin (x) \cos (x) f(x) d x=\int_{0}^{1} \cos ^{2}(x) f(x) d x=1 . \]
Then $S$ is:
- an uncountable set
- a countably infinite set
- a finite and nonempty set
- the empty set