Find the value of `lambda` so that the points `P ,Q ,R` and `S` on the sides `O A ,O B ,OC` and `A B ,` respectively, of a regular tetrahedron `O A B C` are coplanar. It is given that `(O P)/(O A)=1/3,(O Q)/(O B)=1/2,(O R)/(O C)=1/3` and `(O S)/(A B)=lambdadot` (A) `lambda=1/2` (B) `lambda=-1` (C) `lambda=0` (D) for no value of `lambda`