People smoke and drink too much alcohol even though they know *it is* bad for their health. 'It' is always singular, *it are* is not used in any situation. Example: It are a well-known brand. (no) (you can say instead: They are a well-known brand.) It is a well-known brand. (yes)

is it

"It is" Remember, if it's the third person, you have to use "is".

it is