Indicate the main level of ecological study addressed by this question: How does the decline of bee pollinators affect fruit production in apple trees?

The main level is "population".

Explanation:

The main levels of study in ecology are the organism, the population, the community and the ecosystem.

In the case of the question above, we can say that although all levels are included, the research is mainly centered on the "population" level. This is because it seeks to define how a population of pollinating insects (and not just an organism) can interfere in the fruit production of a population of apple trees.

the correct answer is modern american cars are not available due to trade restrictions.

trade between cuba and the united states have serious restrictions. the u.s. imposed an economic embargo on cuba, that among other things means that modern american cars are not available due to trade restrictions.

in response to some governmental actions of the authorities from cuba, president john f. kennedy proclaimed an embargo on trade between both countries in february 1962. the trade embargo remains active until this day and was implemented directly by the departments of commerce and the department of treasury.

in president trump’s administration, there is a national security presidential memorandum that "strengthens the policy of the u. s.", affecting trade with cuba.