Mathematics

Problem

P, Q, R, S

are four coplanar points on the sides

AB, BC, CD, DA

of a skew quadrilateral, then

\frac{AP}{PB} \cdot \frac{BQ}{QC} \cdot \frac{CR}{RD} \cdot \frac{DS}{SA}

equals a. 1 b. -1 c. 3 d. -3