Four of the five parts numbered (a), (b), (c), (d) and (e) are exactly equal. Which of the parts is not equal to the other four? The number of that part is the answer. Choose the correct choice
A) (a + b)(a – 2b)
B) a2 – b(a + 2b)
C) a2 + b2 – ab – 3b2
D) (a + b)(a – b) + ab
E) (a – b)2 + b(a – 3b)
Others equal a2 – ab – 2b2 whereas (d) equals a2 + ab – b2 .   Answer (a + b)(a – b) + ab
<result>D</result>
Four of the five parts numbered (a), (b), (c), (d) and (e) are exactly equal. Which of the parts is not equal to the other four? The number of that part is the answer.