Let $a,b,c$ be distinct non-negative numbers. If the vectors $a\hat{i}+a\hat{j}+c\hat{k}$, $\hat{i}+\hat{k}$ and $c\hat{i}+c\hat{j}+b\hat{k}$ lie in a plane, then $c$ is:

1. A
   The arithmetic mean of $a$ and $b$
2. B
   The geometric mean of $a$ and $b$
3. C
   The harmonic mean of $a$ and $b$
4. D
   Equal to zero