Let \( a,b,c \) be three distinct real numbers, none equal to one. If the vectors \( ai + j + k, \ i + bj + k \) and \( i + j + ck \) are coplanar, then \(\displaystyle \frac{1}{1-a} + \frac{1}{1-b} + \frac{1}{1-c} \) is equal to:
\( -1 \)
\( 1 \)
\( -2 \)
\( 2 \)