In the following excerpt from a literature review, there are 4 sentences. Read the paragraph and identify which sentences are identifying a problem that could be solved with further research: "The process of ploymerisation basically forms a long chain molecule from short chain molecules (Moeller et al. 1989). The obvious benefit of polymerization is that by increasing the size of the molecule, it becomes less soluble and also less able to be carried in the ground water. However, the reaction process means that it will only be effective for organic monomers. As with all in-situ chemical technologies, the chemicals added to the aquifer may react to form toxic substances (Knox et al 1985).