如图，在棱长为2的正方体ABCD﹣A₁B₁C₁D₁中，O是底面ABCD的中心，M、N分别是棱DD₁、D₁C₁的中点，则直线OM（　　）

根据短文内容及所给首字母，将正确的单词写在答题卷上
   James works in a food shop. He helps the shopkeeper. He puts f____  on the shelves every day. Then people come into the shop and buy the food. Today there aren’t any bags of flour(面粉) on the shelf.“James,” says the shopkeeper. “We n____  some flour. Bring some bags of flour to the shelf. Hurry up!”
   James runs to get some bags of flour. They are in another room and they are on the top shelf. The shelf is high up. James is tall b____ he can’t reach the bags.
   James bring two boxes and puts one b____  on the other. Then he stands on them. Now he c____  reach(够到) the bags on the shelf.

下列熟悉的物质中属于纯净物的是(   )

柑橘的果肉酸甜可口，其酸甜的物质主要储存在细胞的（ ）里．

（Ⅰ）求证：S₃，S₉，S₆依次成等差数列；

（Ⅱ）a₇与a₁₀的等差中项是否是数列{a_n}中的项？，如果是，是{a_n}中的第几项？如果不是，请说明理由．