If the direction ratios of two lines are $a_1, b_1, c_1$ and $a_2, b_2, c_2$,then when are these lines parallel?

  • A
    $a_1 = a_2, b_1 = b_2, c_1 = c_2$
  • B
    $a_1a_2 + b_1b_2 + c_1c_2 = 0$
  • C
    $\frac{a_1}{a_2} = \frac{b_1}{b_2} = \frac{c_1}{c_2}$
  • D
    None of these

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