How well do you know your English grammar?

"I ______ my friend John since 2002. We met during our first year in primary school."

"He didn't seem very ________ in what I was saying. He kept checking on his mobile phone."

"A balanced diet is good for ______ health."

What's the correct place to put the comma in the sentence below?

"My parents are on holiday in Hawaii but _______ going to be back next week."