设 X 和 Y 是有限集，而 f: X -> Y 是一个函数。以下哪一项陈述是正确的? _%7C_Question_12_0.jpg)
(一) A
(乙)乙
(C)丙
(四)丁答案： (D)
解释：
Let x = {a, b, c} and y = {1, 2}
A Function f maps each element of x to 1 in y.
f(a)=1 , f(b)=1 , f(c) =1
A = {a, b} B = {b, c}
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A ]
| f(A u B) | = |f({a, b, c})| = 3
| f(A)|+|f(B)| = 2 + 2 = 4 , LHS != RHS.
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B ]
f(A ∩ B) = f({b}) = { 1 }
f(A) ∩ f(B) = {1, 1} ∩ {1, 1} = {1, 1}
LHS != RHS
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C ]
|f(A ∩ B)| = |f({b})| = |{ 1 }| = 1
min{|f(A)|,|f(B)|} = min(2,2) = 2
LHS != RHS
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D ] In a function a value can be mapped only to one value.这个问题的测验