Study the following graph and answer the given question.

The graph shows the time (in minutes) taken by the pipes (A, B), (C, D), (E, F), (G, H) and (P, Q) to fill a tank.

In how many minutes is an empty tank filled completely if pipe D fills it for half the time and then for the other half time, pipes C and D fill it together?

From the graph,

Time taken by the pipe C to fill the tank = 15 minutes

Time taken by the pipe D to fill the tank = 20 minutes

Let the Volume of the tank = 60 units(LCM of 15 and 20)

Volume filled by pipe C in 1 min = 4 units

Volume filled by pipe D in 1 min = 3 units

Volume filled by pipe C and D together in 1 min = 7 units

Since pipe D is working for first half time and pipe C and D together for next half time

Volume filled by pipe D in 1 min + Volume filled by pipe C and D together in 1 min = 3 + 7 = 10 units

In 2 min, combination of these can fill 10 units of Volume

Time required to fill 60 units of Volume = $$\frac{60}{10}\times2$$ = 12 minutes

Hence, the correct answer is Option D