1) If ((→a×→b)×(→c×→d)).(→a×→d)=0 then which of the following is always true ? A) →a,→b,→c,→d are necessarily coplanar B) Either →a or →d must lie in the plane of →b and →c C) Either →b or →c must lie in the plane of →a and →d D) Either →a or →b must lie in the plane of →c and →d Answer: Option CExplanation:((→a×→b)×(→c×→d)).(→a×→d)=0 ([→a→c→d]→b−[→b→c→d]→a).(→a×→d)=0 [→a→c→d][→b→a→d]=0 Either →c or →b must lie in the plane of →a and →d