Which of the following must be non-negative number?  Choices
A) (a-0.9)(a-1)  B) (a+1)(a-1)  C) (a+0.9)(a+1)  D) (a-0.8)(a-0.8)  E) a(a+1)
5-sec approach:   D. (a-0.8)(a-0.8) = (a-0.8)^2.   The square of a number is always non-negative.   Answer: (a-0.8)(a-0.8).
<result>D</result>
Which of the following must be non-negative number?