这句话的结构分析如下：it：形式主语considering ...and introducing...：整段是真正的主语what makes "convenience" foods so popular：是considering的宾语从句一般来说,it作形式主语有3种方式：1） 代替不定式作...我知道it is worth while doing sth。但it is worth doing,doing是真正主语是第一次见到。你肯定吗？It is worth reading a book是正确的？= Reading a book is worth.?在现代英语中，在be worth前使用形式主语it被认为是合习惯的。it is worth doing sth.是很正常的说法。例如以下国外原声例句：It is worth locking these tickets in early and getting a refund later if your plans change. （BBC: Insider flight tips from a seasoned business traveller）worthwhile的意思大致和worth一样，但其后可接动名词或不定式It is worthwhile doing sth.It is worthwhile to do sth.但worth后面只能接动名词。而且，若不用形式主语it而直接用动名词作主语则是错误的，虽然it是代表动名词作主语。如：正：It is worth reading a book. 误：Reading a book is worth.明白。但这里的it显然不是形式主语，因为完全不适合形式主语（可替换）的规则。我也找到了一些例句。It is worth remembering that before the 20th century, all the arts were interactive.值得一提的是，在20世纪以前，所有的艺术都是互动的。In conclusion, it is worth remembering what a treasure trove these tapes truly are. （BBC: The Jackie Kennedy tapes: Sharp, tender and gossipy）Thanks.