Trophic Levels for Human Consumption

Taking a quiz in basic biology and would love some opinions.  Could someone please help with the questions below?

First, does oil bioaccumulate or biomagnify??  I seem to find conflicting answers.  Will it increase at you increase in trophic level? Seems like from what I've read it may but it doesn't follow clear biomagnefication principles like DDT.

1)Based on your data, how the percentage of oil change as you increase in trophic level?   
A) increases
B) decreases
C) stays the same
D) shows no clear trend


Second question below:

Here, I'm leaning towards A because energy is inefficiently transferred as it goes up the food chain/trophic level. 


2) To get the most of the available energy in our food, humans should eat
A) plants (trophic level 1)
B) cows (trophic level 2)
C) tuna (trophic level 3)
D) swordfish (trophic level 4)

Lastly....

3)Buffalo herds move along the prairie eating the grass as they go.  When they have moved on, the grass regrows more quickly and is a better food source for the herbivores on the prairie.  When the buffalo were hunted nearly to extinction, what happened to the other herbivores on the prairie?
A) population density increased
B) population density decreased
C) all herbivores went extinct
D) there was no impact on the ecosystem when the buffalo disappeared

Thanks so much in advance!