A and B separately can build a wall in 12 and 16 days, respectively. If they work for each day alternatively, starting with A, in how many days will the wall be built?

Answer: Option C

2 days → 4 + 3 = 7 units
12 days → 42 units
13 days → 42 + 4 = 46 units
Total time taken = $$13\frac{2}{3}\text{ days}$$