在数列{a_n}中，a₁=1，a_n+1=a_n+1，S_n为{a_n}的前n项和，若S_n=21，则n=____ 

下列现象属于生物的生命现象的是（　　）

    Mr. and Mrs. Su don't often go out in the evening, but today they go out for a walk after supper. They go to the park and have a good time there. On their way home, Mr. Su says to Mr. Su, "Look! A woman is running in the street and a man is running after her." Mr. Su says, "What's the matter? Let's go and help her."

    Now the woman gets near them. Mr. Su asks, "Shall we help you?" "No, thank you," the woman says, "My husband（丈夫） and I are running home, and the slower (较慢的) one does the housework."

1911年爆发的辛亥革命，它虽然没有改变中国半殖民地半封建的社会性质，但南京临时政府采取的许多措施，使人们的精神面貌发生了很大变化。出现了女性积极参加社会政活动，如女子北伐军，女子参政团，女子自由党等现象。这种现象反映了（  ）