想大神解释下这道economics 题目

if both apples and oranges are economic goods a consumer will necessarily refer a

a) combination of 10 apples and 10 oranges to one of 12 apple and 12 oranges
b) combination of 10apples and 50 oranges to one of 11 apples and 8 oranges
c) combination of 100 apples and 9 oranges to one of 10 apples and 10 oranges
d) combination of 10 apples and 10 oranges to one of 9 apples and 8 oranges

answer is d)

不明白为什么答案是d
是这么得出来的？