If f and t are integers and f^2 – t^2 is an odd integer, which of the following must be an even integer?
I. f + t + 2
II. f^2 + 2ft + t^2
III. f^2 + t^2  Choose the correct choice from the following:
A) None B) I only C) II only D) III only E) I, II, and III
f^2–t^2 is an odd integer means that either f is even and t is odd or f is odd and t is even.   Check all options:   I. f + t + 2 --&gt; even+odd+even=odd or odd+even+even=odd. Discard;   II. f^2 + 2ft + t^2 --&gt; even+even+odd=odd or odd+even+even=odd. Discard;   III. f^2 + t^2 --&gt; even+odd=odd or odd+even=odd. Discard.   Nonenswer: None.
<result>A</result>
If f and t are integers and f^2 – t^2 is an odd integer, which of the following must be an even integer?
I. f + t + 2
II. f^2 + 2ft + t^2
III. f^2 + t^2