Which of the Following Sentences Is an Example of Mixed

Multiple Choice

Which of the following sentences is an example of mixed construction?

A) I learned Spanish while my friend tried to learn Chinese.
B) Alex was busy preparing for his exams, so his wife went shopping alone.
C) The children are busy studying because they have a test tomorrow.
D) The teacher graded the test and students were given the scores.
E) After the test, the students went to watch a movie.