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There are many benefits to timing your practice, including:
(1) Either both lines intersect the +x axis or the -x axis. Insuff.
(2) One of them intersects the +y axis and the other one the -y axis. Insuff.
(1&2) The only feasible configuration is: one line intersects the +x axis and the +y axis (and has a positive slope), and the other line the +x axis and the -y axis (and has a negative slope). Therefore, the product of their slopes is negative. Suff. C.