The reading states that zebra mussel has long been spreading out from its original habitats and has now reached parts of North America. therefore it poses a serious threat to freshwater fish populations in all North America and provides three reasons for support. However, the professor explains that zebra mussel is not likely to the serious threat to the freshwater fish population and refutes each of the author's reasons. First, the article claims that it is possible that zebra mussel spread to North America from Europe by human transportation.The professor refutes this point by saying that in the reading author refers to this that mussel collects in the ballast water of the ship. but when ships arrive in North America, the ballast
