Which sentence combines these two sentences and includes a relative clause?
A. The twins pretended to be each other.
B. The switch was a huge mistake.
C. The twins pretended to be each other, a huge mistake.
D. The twins pretended to be each other because they made a huge mistake.
E. The twins pretended to be each other, which was a huge mistake.
F. The twins pretended to be each other, and it was a huge mistake.