Read the following excerpt from a student essay. How can the writer best improve his or her grammar and mechanics?
Found in the icy waters of the Arctic Ocean the narwhal is a species of whale with a long tusk. Generally, only males have tusks but occasionally females may develop them. The tusks grow up to nine feet long and weigh more than twenty pounds. Researchers aren't exactly sure why narwhals grow these tusks. They might be used to fight other males, to impress females or to help the creatures detect changes in their environment.

 - by fixing misplaced modifiers
 - by fixing run-on sentences
 - by adding missing commas
The writer could best improve his or her grammar and mechanics by adding missing commas.
For example, the writer could add commas in the underlined sentences to follow an introductory phrase, to separate the clauses of a compound sentence, and to separate items in a series.
Found in the icy waters of the Arctic Ocean the narwhal is a species of whale with a long tusk. Generally, only males have tusks but occasionally females may develop them. The tusks grow up to nine feet long and weigh more than twenty pounds. Researchers aren't exactly sure why narwhals grow these tusks. They might be used to fight other males, to impress females or to help the creatures detect changes in their environment.