17. Star S1 is moving away from us at a speed of 0.8c (c is the speed of light). Star S2 is moving away from us in the opposite direction at a speed of 0.5c. The speed of S1 as measured by an observer on S2 is:
(A) 0.21c
(B) 0.50c
(C) 0.93c
(D) 1.3c
(E) 2.2c

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