If `P_1:vec r.vecn_1-d_1=0` `P_2:vec r.vec n_2-d_2=0` and `P_3:vec r.vecn_3-d_3=0` are three planes and `vec n_1, vec n_2` and `vec n_3` are three non-coplanar vectors, then three lines `P_1= ` `P_2=0`; `P_2=``P_3=0` ; `P_3=` `P_1=0` are
a. parallel lines
b. coplanar lines
c. coincident lines
d. concurrent lines