For all positive values of \(a\) and \(b\), let \(a \odot b\) be defined by \(a \odot b = \frac { ab } { a + b } \)? If \((10 \odot 10)^2 - (8 \odot 8)^2 = k\), where \(k\) is a constant, what is the value of \(k\)?
(A)1 (B)2 (C)4 (D)6 (E)9
E