Answer :
The most difficult planets for Mark to learn will probably be Jupiter and Saturn. These planets are much larger and more massive than the others, making them more difficult to remember.
Additionally, they are located further from the sun, making them less likely to be remembered in the correct order. Finally, they have many moons, which can make them difficult to keep track of.
Jupiter and Saturn are also important because they are the gas giants. Gas giants are huge planets made mostly of gas and dust. They are very different from the other planets in the solar system, and they can be very difficult to understand. Additionally, Jupiter and Saturn have a lot of moons, which can be difficult to keep track of.
Overall, Jupiter and Saturn are the most difficult planets for Mark to learn, but they are also the most important. If he can remember the order of the planets and the gas giants, he will be able to understand the solar system as a whole.
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