Criminal Sentence 58: Who's or Whose?

From a book I'm reading:

"Who's line is that?"

If you spell out the contraction, this question becomes "Who is line is that?" and that doesn't make any sense.

These pairs are easy to confuse if you're not careful:

you're, your
it's. its
who's, whose
they're, their, there

Spell out your contractions to check if your right. (Did you catch that?)