11.已知无穷实数列{an}\{a_{n}\}{an}的前nnn项和为SnS_{n}Sn.若数列{Sn}\{S_{n}\}{Sn}既有最大项,也有最小项,则在:①"a1>0a_{1}>0a1>0且数列{∣an∣}\{\vert a_{n}\vert \}{∣an∣}严格减"和②"a1<0a_{1}<0a1<0且数列{∣an∣}\{\vert a_{n}\vert \}{∣an∣}严格增"中,{an}\{a_{n}\}{an}可能满足的条件是((( )))