1. a, b 都對。burden 也有 responsibilty 的意思(字典有
http://www.ldoceonline.com/dictionary/onus 有列出「題目的那一個」固定用法。你換查 burden ,仍是有 burdenon,或者該說這字能用的範圍/搭配用字/句型較廣換用Google Books查: books.google.com 輸入下列兩者(含前後雙引號):"the burden is on the teacher to""the onus is on the teacher to"兩者都點最後一頁(多次)。我這邊最後顯示前者是16筆,後者是11筆 結果(都很少也) 很接近一個實例:
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