请问,against在这里是什么词性,什么意思

prep.介词
例句：He was elected by a majority of 30 votes against 15.
他以30票对15票的多数当选.
例句:Many bills were entered against her accounts.
许多账单被记入她的账户.
例句:The waves were dashing against the shore.
波涛拍打着海岸.
例句:We are all taking medicine against the cold.
我们都服药以防感冒
例句:I checked my watch against GMT.
我以格林尼治时间对表.
例句:We must finish our work against Saturday.
我们必须在星期六之前做完我们的工作.
希望有用、、