事实上，只须两个字母就已经足够：delta和eta。

Actually, just two letters will do: delta and eta.

youdao

什么字由五个字母组成，减去两个字母只余一？

What word of five letters has only one left when two letters are subtracted?

youdao

You are required only to put the one or two-letter chemical abbreviation, the symbol.

你们只需要,用一两个字母来代表一个化学元素。

麻省理工公开课 - 固态化学导论课程节选