My grandparents wrote letters to keep in touch with others ______ they were young.
考查连词的用法。
四个选项的含义分别是：when当……的时候；since自从；after在……之后；before在……之前。结合本题语境可知，当我爷爷奶奶年轻的时候，他们写信和别人保持联系。表示“当……的时候”要用连词when，故答案为A。
We gave away ______ books to the school library last year.
考查thousand的用法和搭配。
six thousand意为“六千”，当thousand和具体的数字搭配时，要用的thousand的单数形式，而且其后不加of。故选B。
—Can I help you,sir?
—Yes,please. I am ______ a blue T-shirt.
考查look短语用法。
look up查阅；look for 寻找；look after 照顾；look through 浏览；根据本题语境可知，那位男士正在“寻找”意见蓝色的T恤，故选B。