96台大電機 (題庫T2 第2-40頁)

96台大電機 (題庫T2 第2-40頁)
How many of following function have inverse?
(1) f:Z→Z where f(x)=x mod 10
(2) f:A→B where A={a,b,c} B={1.2.3} and f={(a,2),(b,1),(c,3)}
(3) f:R→R where f(x)=3x-5
(4) f:R→R where f(x)= 2x(取floor )
(a)0 (b)1 (c)2 (d)3
ANS:©

想請問一下這兩個是(1)跟(4)嗎?~!謝謝大家!!