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.

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紫色的蛇不会减法，而不会减法的蛇也不会加法，所以紫色的蛇不会加法。

另一方面，开心的蛇都会加法，所以开心的蛇不可能是紫色的。

所以正确答案是 D。

Note that the third condition ensures that purple snakes can't add.

We also know that all happy snakes can add, which means that happy snakes can't be purple as well.

Thus, D is the correct answer.