For \( x \ge 0 \) the least value of \( K \), for which \( 4^{1+x}+4^{1-x},\; \dfrac{K}{2},\; 16^{x}+16^{-x} \) are three consecutive terms of an A.P., is equal to:
\( 8 \)
\( 16 \)
\( 10 \)
\( 4 \)