The reason that answer choice C is correct is because the first sentence (the one under question) is a sub-conclusion. It is actually not very common for the first sentence of the stimulus to be a sub-conclusion, but we need to be flexible with our approach and make sure we are considering the evidence presented to us.
The first sentence says: "The best jazz singers use their voices much as horn players use their instruments."
The second sentence says: "The great Billie Holliday thought of her singing voice as a horn..."
The second sentence is actually true support (premise) for the first sentence (sub-conclusion).