Put the following four sentences into the right order so that it conforms to the rhetorical structure of the opening of a research paper?
a.  In recent years, there has been an increased awareness of the potential impact of pollutants such as heavy metals.
b.  Moreover, the traditional methods for treating aqueous streams containing metal contaminants are expensive and can have inadequate facilities.
c.  The use of biological materials is one such technology which has received considerable attention.
This has led to the use of alternative technologies.