There was clearly nothing to do but sit down and cry. So Della cried. Which led to the thought that life is made up of little cries and smiles, with more little cries than smiles.
source:A Special Story for Christmas: 'The Gift of the Magi'
I just can't understand the meaning of the bold sentence above, and I can't convince myself to accept that sentence as clarified —— especially the use of "which"! For there is nothing to choose from!
What's more, it's not a question sentence(without question mark)!
Why don't use "what",which seems much more natural and clarified to me?