I'm doing Duolingo Spanish. The other day, I had the sentence

> 
> We are not doing it
> 
> 
> 

To which I gave the answer

> 
> No la hacemos
> 
> 
> 

This was marked wrong, with the correct solution of 

> 
> No lo hacemos
> 
> 
> 

I admit I'm not real clear on the lo, le, la difference (which Duolingo never explains), even after looking at a couple explanations online.

So, is la incorrect here (and why?), or is Duolingo (for rejecting it; they usually take most reasonable translations)? 

![lo vs la](https://i.stack.imgur.com/V2iQs.png)