1. a first-grade teacher use ten flash cards, numbered 1 through 10, to teach her students to order numbers correctly. she has students choose four flash cards randomly, then arrange the cards in ascending order. one day, she removes the cards numbered 2 and 4 from the deck of flash cards. on that day, how many different correct arrangements of four randomly selected cards are possible?
选项A70 B210 C336 D840 E1680
2. if n is a positive integar and n²is divisible by72,then the largest positive integar that must divide n is
