Fill in the blanks with a preposition to form the right phrasal verb.
add up = find the sum of
answer back = reply rudely, argue
ask after = inquire about someone's health
ask out = invite out
back up = support
back out = withdraw
be up to = be engaged in sth wrong
beat up = injure (usually in a fight)
bend down = stoop
blow up = 1. explode 2. enlarge