we know that gravity is strongest when two large objects are near each other--so, when the asteroid reaches point C (which is shown to be closest to the sun), the pull of gravity will be greatest.
We also know that the attraction/pull of gravity slows down objects, so, the asteroid will be going slowest at its point of strongest gravity--which we have determined to be point C
hope this helps! let me know if you need more explanation, have a lovely day :)
