1-C. He always manages to sell more than his colleagues do because of his congenial personality.He always manages to sell more than his colleagues do because of his congenial personality.
2-C. Some of his students have complained about his jokes being a waste of time, His colleagues also have advised him not to spend so much time writing jokes.Some of his students have complained about his jokes being a waste of time, His colleagues also have advised him not to spend so much time writing jokes.