[離散] 99台大電機

If f : X→Y is a one-to-one and onto function, and Y is a proper subset of X, which of the following statement is correct?

(a) X must be countably infinite
(b) Y must be countably infinite
(c) X must be infinite
(d) The cardinality of X is lager than Y
(e) None of the above

答案是c
想請問這題在考什麼概念呢？為什麼答案是c？完全沒有頭緒…
感謝！