Consider the three planes     P1:3x+15y+21z=9      P2:x−3y−z=5                   P3:2x+10y+14z=5            Then , which one  of  the following is true ?

The direction ratios of normal to the planeax+by+cz+d=0 are a,b,cThe direction ratios of normal to the planeP1:3x+15y+21z=9 are 1,5,7The direction ratios of normal to the planeP2:x−3y−z=5 are 1,−3,−1The direction ratios of normal to the planeP3:2x+10y+14z=5 are 1,5,7Observing the above three planes, direction ratios of normal lines of the planes P1  and P3 are proportional.  Hence, the planes P1,P3 are parallel.