单选题 .  ①Arctic sea ice comes in two varieties. ②Seasonal ice forms in winter and then melts in summer, while perennial ice persists year-round. ③To the untrained eye, all sea ice looks similar, but by licking it, one can estimate how long a particular piece has been floating round. ④When ice begins to form in seawater, it forces out salt, which has no place in the crystal structure. ⑤As the ice gets thicker, the rejected salt collects in tiny pockets of brine too highly concentrated to freeze. ⑥A piece of first-year ice will taste salty. ⑦Eventually, if the ice survives, these pockets of brine drain out through fine, veinlike channels, and the ice becomes fresher; multiyear ice can even be melted and drunk.35.  The passage mentions which of the following as being a characteristic of seasonal ice? ______