Which of the following is an explanatory thesis?
a. Music streaming services can be a powerful promotional tool for artists if they use the sites wisely.
b. Online music streaming is a beneficial element in the music industry; it decreases music piracy and increases the exposure of new artists and styles.
c. Streaming music services are unfair to musicians: they reduce royalties and impact how artists make a living.
d. Some contend that streaming music online is unfair to musicians, while proponents of the technology focus on the benefits of these services to consumers.