Please help Which condition will calls particles to diffuse faster across a membrane

A: an increase in size of the particles
B: The same amount of particles on each side
C: Cooling of the temperature
D: A steep concentration gradient

the answer is minerals.

the answer is b because, investigating whether another species of moth also drinks tears might be interesting, but it would not directly explain why this species of moth does.

to explain why this species of moth drinks tears, the scientist could ask: "what nutrients that moths need are in elephant tears? "

the scientist could experiment with moth repellent or oter ways to keep the moths away, but that would not answer his original question.