We often say that something is “as light as air”, but air is＿＿＿light. 

. Come _____. I’ll introduce my _____friend, Bob, to you.

Most of my friends use their cars ________for their job.

The newly-published book is written _________ for children, which is also popular with most parents.

—Must I turn off the gas after cooking ? --Of course . You can never be ___ careful with that.