介词by后面的复数名词能不能加s
9年级的英语书上册中（仁爱的）
早77页有这么一句话,“Spaceships are mainly controlled by computers." 可是93页又有一句话是" All the people travel by spaceship in the movie ." computer 和spaceship都是可数的名词啊.并且第5页中也有这个情况,“People kept in touch with their friends and relatives far away by letter.” 呃,letter也是可数名词啊,可是为什么BY后的可数名词有时候加S有时候则不加呢.
请英语高手赐教.我不甚受恩感激哈.