If a block has a density of 1.8 g/ml, will it float?

NO, the block with density 1.8 g/ml will not float.

Explanation:

Density is the relationship between volume and mass, whereas buoyancy is the ability of an object to float. Its buoyancy influences the density of an object.  

For an object float its density should be less than the density of water.If the density is higher , then it will not be able to displace equal amount of water causing the object to sink.

Here the block with density 1.8g/ml would sink since its density is greater than the density of water. The density of water is 1g/cm cube,

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.