Simplify the following text for a Grade 7 native English-speaking person.
Original: The quintets use two violas , which frees the instrument ( especially the first viola ) for solo passages and increases the variety and richness of the ensemble . Mozart also wrote for the viola in his Sinfonia Concertante in which the two solo instruments , viola and violin , are equally important .
Simplified: The viola can also be a solo instrument , but is mostly used in an orchestra .