For 1a, since it is going down, the vertical lines signify that there are less raisins (and he is hopefully eating them)!. For 1b, it's entirely possible that Vincent could eat a different number of raisins, so that means that the line could be longer or shorter. Regarding 1c, if all the raisins were eaten, then there would be no mass as raisins take up mass. As that is not the case, there are therefore some raisins left.
For 2a, since a change in the y value (it goes up or down) means that the volume of juice changes, since there is no change in the y value when the line is horizontal that means that the amount of juice is not changing and Ellie is therefore not drinking from it at that time. The answer for 2b would be that it goes at an angle due to that when Ellie drinks from the juice box, the amount of juice decreases and is shown with 'Volume in milliliters'. Next, since it decreases, it therefore cannot stay in a straight line and therefore goes at an angle