onewho是什么意思

有这样一道题：
He is a hard-working student, _________ who will be successful in whatever career he chooses.
A. that                      B. he                        C. one                      D. which
此题答案选C此题答案选，one 相当于 a student。类似地，以下各题也选one，它们分别相当于 a table, a break, a question, a Mexican carpet, a moment：

(1) A table made of steel costs more than _________ made of wood.
A. one                      B. it                          C. those                    D. which

(2)“Why don’t we take a little break?” “Didn’t we just have _________?”
A. it                   B. that                      C. one                      D. this

(3) The question is _________ of great importance.
A. that                      B. it                          C. one                      D. what

(4) She wants a Mexican carpet, but she can’t afford to buy _________.
A. one                      B. it                          C. them                    D. the one

(5) Meeting my uncle after all t4hese years was an unforgettable moment, _________ I will always treasure.
A. that                      B. one                      C. it                         D. what