extract from json

[teddybubu at gmail.com]

I can't find any example on how to do this.
I have a json file like so:
{"bostock":[{"url":"http://bl.ocks.org/mbostock/9360565","title":"titleplaceholder","date":"dateplaceholder"},
{"url":"http://bl.ocks.org/mbostock/9265674","title":"titleplaceholder","date":"dateplaceholder"},
{"url":"http://bl.ocks.org/mbostock/9265467","title":"titleplaceholder","date":"dateplaceholder"},
{"url":"http://bl.ocks.org/mbostock/9234731","title":"titleplaceholder","date":"dateplaceholder"},
{"url":"http://bl.ocks.org/mbostock/9232962","title":"titleplaceholder","date":"dateplaceholder"},

this goes on for more than 700 entries. only thing unique is the number at the end of the url. I am going to load the url in python, get the date and title and write it in the json itself. 
Right now I am stuck on just reading the url in the json. Here is my code:

import json

with open("bostock.json") as json_file:
    json_data = json.load(json_file)
    print(json_data)

I have tried json_data[0], json_data.url and a few others I forget right now and it does not seem to work.  

I have already figured out how to get the title and date.
First things first: How can i just get the url for each line of the above json file?