In an A.P., the sixth term \( a_6 = 2 \). If the product \( a_1 a_4 a_5 \) is the greatest, then the common difference of the A.P. is equal to:
\( \dfrac{5}{8} \)
\( \dfrac{2}{3} \)
\( \dfrac{3}{2} \)
\( \dfrac{8}{5} \)