Because Hilary is trying to calculate her bone density change from year to year, then, it is normally done once a year, preferably with twelve months gap every calendar year. So, for example, Hilary may try to calculate her bone density on, say June 1 of this year and will then calculate the bone density again on June 1 of the next year. Thus, she wont be calculating her bone density any time in between and thus there will be a gap. This gap will make the function that would best model Hilary's bone density to be a Discrete Model. Because there are gaps and the calculations are not done at every continuous instant. Thus, the second option is the correct one.