1. a, b 都对。burden 也有 responsibilty 的意思(字典有
http://www.ldoceonline.com/dictionary/onus 有列出“题目的那一个”固定用法。你换查 burden ,仍是有 burdenon,或者该说这字能用的范围/搭配用字/句型较广换用Google Books查: books.google.com 输入下列两者(含前后双引号):"the burden is on the teacher to""the onus is on the teacher to"两者都点最后一页(多次)。我这边最后显示前者是16笔,后者是11笔 结果(都很少也) 很接近一个实例:
http://tinyurl.com/gmckhdn "... burden ..to change the teaching."
http://www.dictionary.com/browse/onus?s=t 字源本身就是"burden"的意思