Let \( a_1, a_2, a_3, \ldots \) be a G.P. of increasing positive terms. If \( a_1 a_5 = 28 \) and \( a_2 + a_4 = 29 \), then \( a_6 \) is equal to:
\( 812 \)
\( 628 \)
\( 784 \)
\( 526 \)