Is it a cello? Is it a violin? What is it?

Silence fills the room as the orchestra enters onto the stage. One of the violinists, or so the people think, comes to the front of the stage with his instrument. The conductor comes on and takes a bow. Turning to his orchestra, he glances over at the soloist standing just behind him on the left. Then he raises his baton and the music begins....
After a small introduction by the orchestra, the violinist (again, so the audience thinks) starts playing. The instrument's voice fills the room. The rich and mellow sound flies straight to their ears as the bow moves up and down and fingers dance along the fingerboard. The audience looks into their programs and sees that the soloist is playing the viola.

Well, now the question I need to answer is: What on earth is the viola?

The viola is a stringed instrument, sort of like the violin and the cello. The viola is slightly bigger in size compared to the violin and has different strings. The violin has a G string, a D string, an A string, and an E string. The viola has slightly different strings. It has a C string, a G string, a D string, and an A string; the same strings as the cello. The difference between a cello and a viola is slightly obvious, as it is held much differently and is much smaller in size. The cello is an octave below the viola. The viola has different playing techniques from those of the violin. Reasons being that the instrument is bigger and the bow is heavier than that of the violin.

The instrument is very not well-known to people. But it is a treasure just waiting to be heard..... One that only that string players have found...