There are two marks between each two adjacent integers. There are three sets of adjacent integers between 13 and 16, so there are 6 marks between the integers. Since the question asks for the number of marks between 13 and 16, the marks on 13 and 16 are not included. So the 2 marks at 14 and 15, added to 6, yields 8, which is D. If it helps to do the problem visually, draw out the line and the marks and then count them up.