Solution: Analysis: No matter whether L 1 and L2 are the same plane straight lines or different plane straight lines, if the straight lines are common vertical lines, the common vertical lines must be tangent vectors vt 1 and vt2 which are perpendicular to the two straight lines at the same time. That is, the tangent vector of the common vertical line vt=λvt 1xvt2. For the question of choosing the answer, one is to see whether the tangent vector of the common vertical line meets the requirements, and the other is to see whether the common vertical line is on two straight lines. Vt = λ vt 1xvt2 = λ {2,-1, 1} x {-3,2,4} = λ {-6,-1 1. Please check whether you have written the wrong question; If this is what I said, then the tangent vector of the common vertical line vt = λ {2,5,1}; Only answer (c) matches it; Then choose answer (c). If there is no problem with your question, then the answer has no required common vertical line.