When do you use can vs could? I always get confused here.

Read this passage: many manufacturers prefer plastic as a material for containers because it can be molded into many shapes. but plastic is not very strong. it must be fused with glass or steel for strength. it is also not very durable. even the strongest plastic can break under enough pressure. the biggest disadvantage to using plastic is that it does not decompose. while plastic can be recycled, doing so is tedious and difficult. while it may be more expensive and difficult to work with, aluminum is a much stronger building material with far greater tensile strength. it conducts electricity because it is metal. best of all, aluminum is very easy to recycle. even if it is thrown away, it decomposes much more quickly than most manufacturing materials. how does the compare-and-contrast organizational pattern of the passage support the author's purpose? a. the passage gradually leads up to the point about decomposition to inform the reader about the importance of recycling plastics. b. the passage lists the advantages and disadvantages of each material to make a case for the use of plastic and not aluminum in containers. c. the passage begins with a description of plastic's problems in order to call attention to the good qualities of aluminum. d. the passage shows the superiority of aluminum in order to make a case for the production of stronger plastics.