They quickly brought down the age at marriage for both men a

They quickly brought down the age at marriage for both men and women and brought the birth rate to a twentieth century height after more than a hundred years of a steady decline
这里有一个错误我不知道是怎么回事,就是a steady decline这里的a要去掉,答案是这么说的,而且还说decline是一个不可数名词,应该去掉冠词a,但我已经查过了,decline在作“下降”解时是可数的,不知道这里是怎么回事