Which organism might be at the top of the food chain in a deciduous forest?

Ineed to know why d is the correct answer asap

Pentane with molecular formula c5h12, exists in three isomeric forms. one shows linear carbon chains, another has one -ch3 groups present on the third carbon atom, and the third has two -ch3 groups present on the second carbon atom. what types of isomers are these? a. geometric isomers b. structural isomers c. halotropic isomers

Crossing over is a method of (blank)?