Question

Tom 收藏了 $13$ 条蛇，其中 $4$ 条是紫色的， $5$ 条是开心的。他观察到

• 所有开心的蛇都会加法，

• 没有紫色的蛇会减法，并且

• 所有不会减法的蛇也不会加法。

关于 Tom 的蛇，可以推出下列哪个结论？

Tom has a collection of $13$ snakes, $4$ of which are purple and $5$ of which are happy. He observes that

• all of his happy snakes can add,

• none of his purple snakes can subtract, and

• all of his snakes that can’t subtract also can’t add.

Which of these conclusions can be drawn about Tom’s snakes?

紫色的蛇会加法。

Purple snakes can add.

紫色的蛇是开心的。

Purple snakes are happy.

会加法的蛇是紫色的。

Snakes that can add are purple.

开心的蛇不是紫色的。

Happy snakes are not purple.

开心的蛇不会减法。

Happy snakes can’t subtract.

串联这些蕴含关系：紫色 $\Rightarrow$ 不会减法 $\Rightarrow$ 不会加法

Chain the implications: purple $\Rightarrow$ can’t subtract $\Rightarrow$ can’t add

开心的蛇会加法，但紫色的蛇不会加法；比较这两个事实

Happy snakes can add, but purple snakes cannot add; compare these two facts

紫色的蛇不会减法，而任何不会减法的蛇也不会加法。所以每条紫色的蛇都不会加法。

每条开心的蛇都会加法。由于紫色的蛇不会加法，没有开心的蛇可能是紫色的；也就是说，开心的蛇不是紫色的。

因此，正确答案是 D。

A purple snake cannot subtract, and any snake that cannot subtract also cannot add. So every purple snake cannot add.

Every happy snake can add. Since purple snakes cannot add, no happy snake can be purple; that is, happy snakes are not purple.

Thus, the correct answer is D.