Definition: You're given a sentence and your task is to classify whether the sentence is acceptable or not. Any sentence which is grammatically correct, has a naturalistic text, is written by a native speaker and which minimizes superfluous content is acceptable, otherwise unacceptable. If the sentence is acceptable then write "acceptable", otherwise "unacceptable".

Positive Example 1 -
Input: Our friends won't buy this analysis, let alone the next one we propose.
Output: acceptable

Positive Example 2 -
Input: The knee replacement candidate groaned up the stairs.
Output: acceptable

Negative Example 1 -
Input: They made him to exhaustion.
Output: acceptable

Negative Example 2 -
Input: They made him into a monster.
Output: unacceptable

Now complete the following example -
Input: Why haven't any books been returned?
Output: acceptable