How Python programs parse strings

I call a function , The returned string is as follows ：
{
"nbest" : [{
"sentence" : " The only one is a broken car, which is all over the gully, drilling around and swearing. You get on first, only one jeep is too poor, and this jeep is still tied to Li Dakang's buttocks ."
}],
"type" : "final_result"
}

In the string sentence Corresponding content , How do I get ？
I need to put “ The only one is a broken car, which is all over the gully, drilling around and swearing. You get on first, only one jeep is too poor, and this jeep is still tied to Li Dakang's buttocks .” Read out , Assign to a string Variable .
Please tell me how I should do ？