If the angle between the lines whose direction cosines are ( -frac { 2 }{ \sqrt { 21 } } ,frac { C }{ \sqrt { 21 } }

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Q 2417691580.     If the angle between the lines whose direction cosines are ( -frac { 2 }{ \sqrt { 21 } } ,frac { C }{ \sqrt { 21 } } ,frac { 1 }{ \sqrt { 21 } } ) and ( frac { 3 }{ \sqrt { 54 } } ,frac { 3 }{ \sqrt { 54 } } ,frac { 6 }{ \sqrt { 54 } } ) is frac { \pi }{ 2 }`, then the value of C is
EAMCET 2016
A

6

B

4

C

-4

D

2

HINT

(Provided By a Student and Checked/Corrected by EXXAMM.com Team)

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