You are right

"Sincerely, John" is the formal way to end a cover letter.

Sincerely means honestly and without pretending or lying. 

Question 5: When you've finished writing your cover letter, it's best that you get a friend to "proofread" for you. What does "proofread" mean? 

a. To spit on it

b. Ensure your cover letter is long enough

c. Check for spelling and grammatical errors 

d. Add illustrations