介詞by后面的復(fù)數(shù)名詞能不能加s
9年級的英語書上冊中（仁愛的）
早77頁有這么一句話,“Spaceships are mainly controlled by computers." 可是93頁又有一句話是" All the people travel by spaceship in the movie ." computer 和spaceship都是可數(shù)的名詞啊.并且第5頁中也有這個情況,“People kept in touch with their friends and relatives far away by letter.” 呃,letter也是可數(shù)名詞啊,可是為什么BY后的可數(shù)名詞有時候加S有時候則不加呢.
請英語高手賜教.我不甚受恩感激哈.