Given the Question and Answer pair, detect whether the answer is acceptable or not. Denote acceptable answer as "yes", otherwise denote as "no". An acceptable answer is consistent with the context in question, detailed, long, and uses general knowledge of English. Answers that are not definite and suggest a variety of options are not acceptable answers.

Q: Question: Is singular correct, or is plural of "credential" correct for a single username & password pair? 
Answer: This isn't a question you can answer by thinking about the grammar - it's just a case of conventional usage. I've previously worked in the tech industry, and we would always say credentials, never credential. That would still be true even if the credentials were a single item like a certificate. [Edit to add: but Samuel has provided a counter-example to this in a comment on the OP.] In wider English, credential is valid - it's used for example in this Wikipedia article - but it's still more common to use the plural (see Oxford Dictionaries for confirmation). For example, I could say that I have presented my credentials to someone if I have shown them my passport, even though I've only shown them one physical item.

A: yes
****
Q: Question: I am referring to someone who makes people 'buy' what he says and gets things done by them through nothing but clever speech something one can describe as crafty or beguiling, but not fraudulent. I pondered over con man and con artist for a while but those terms seem to hold a hint of crime, so those terms won't fit in. I am wondering if there is a more suitable phrase or word. 
Answer: Not quite a single word, but silver tongued probably fits.

A: yes
****
Q: Question: What can I call someone (a friend) who is always asking for favours such as: if you are going downtown, can you buy this for me please or, can you lend me your car for an hour or, if you have some spare time, can you post this letter for me please because I am so busy today He doesn't seem to realise or, pretends not to, that there is a limit to asking for help or favours from other people. 
Answer: You can say that your friend is "using" you, as to describe their behavior of taking advantage of you, thus making them a "user.".

A:
no
****