When arranged, it means a up and down, and when combined, it means c up and down.

The so-called arrangement is to select n (where n stands for 1) from the total number (here 4) for sorting. Consider sorting.

In other words, it must be sorted out when it is taken out, and there can be no repetition or omission.

Combination means that no matter what sort, only n numbers are taken out of the total. In other words, taking n from the total is over.