Well, most slang words that have been used for a long period of time have been added to the dictionary, but then again, some other slang words have not. So, it really depends on the word.
But, for a straightforward answer i would say yes, just because some slang words are in the dictionary.
~Hope this helped
Details will be the answer....
Pretty sure the answer is stiffness. Hope this helps.
The answer is D: add -s.
For instance, the plural of cat is cats, you have one dog but two dogs, and so on.
The answer is C), playing a simple scale.
The sentence should be this: She demonstrates how to tune her instrument and she also played the simple scale.