The rate of flow of water in a capillary tube is determined by the pressure difference between the two ends of the tube, as well as the dimensions of the tube itself. In this case, we are comparing the rate of flow in two capillary tubes with different lengths and radii, but the same pressure difference.

Let's consider the first capillary tube with length l and radius r. The rate of flow of water in this tube is denoted as V.

When we compare this with the second capillary tube, which has a length of 2l and a radius of 2r, we need to determine how the rate of flow in the second tube is related to the rate of flow in the first tube.

By comparing the dimensions of the two tubes, we can see that the length of the second tube is twice that of the first tube (2l = 2 * l), and the radius of the second tube is twice that of the first tube (2r = 2 * r).

The rate of flow of water through a capillary tube is inversely proportional to the length of the tube and directly proportional to the radius of the tube. This means that if the length of the tube is doubled, the rate of flow is halved, and if the radius of the tube is doubled, the rate of flow is doubled.

So, in the second capillary tube, the length is twice that of the first tube, which would result in a halving of the rate of flow. However, the radius of the second tube is also twice that of the first tube, which would result in a doubling of the rate of flow.

Therefore, the overall effect is that the rate of flow in the second tube is 2 times the rate of flow in the first tube. Mathematically, we can express this as:

Rate of flow in second tube = 2 * (Rate of flow in first tube)
Rate of flow in second tube = 2V

Hence, the correct answer is option C: 8V, which means that the rate of flow in the second capillary tube is 8 times the rate of flow in the first capillary tube.