I'm having a lot of trouble with this problem. 
![enter image description here](https://i.stack.imgur.com/28pGB.png)

A - I think is linear by definition  

B - Now, I don't think this is linear, but I marked it anyway because I was unsure (you're multiplying)  

C - I am on the boundary with this too, but I think this IS linear. Never mind my marking in the image.  

D - This is linear  

E - This too I think is linear by definition  

F - This is obviously not linear because it is multiplying. 

But, the program says I'm wrong.