题目

根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项多余选项。Atravel journal is one of the best ways to keep a long voyage in memory. Manydetails that don't seem important while you are writing your travel journaloften tum out to be so memorable. When you are on a busy journey, it is easy to forget to write. You maypromise yourself that you will write later, but that almost never happens. Takea few minutes every day and put down your memories. .Useyour down time for your travel journal. There is a surprisingamount of sitting around whi已知{an}为等差数列，a1+a3+a5=105，a2+a4+a6=99，以Sn表示{an}的前n项和，则使得Sn达到最大值的n是（ ）A．21B．20C．19D．18