**Concept:** **The question tests on the basics of Rates(Work and Time)**

**Solution:**

If C alone can finish a work in 60 days, then work done in 1 day = 1/60

If C does half the work as A and B in 1 day, then the work done by A and B in 1 day = 2 * 1/60 = 1/30

Working together, the total work done in a day = 1/30 + 1/60 = 3/60 = 1/20

Time taken to finish the work together (flipping numerator and denominator) = 20 days**Option C**