Answer:
Mount St. Helens is best known for its major eruption on May 18, 1980, the deadliest and most economically destructive volcanic event in U.S. history. ... The 1980 eruption disrupted terrestrial ecosystems near the volcano.
Explanation:
The bolded pronoun in the given sentence is the word "which". This pronoun is an example of an interrogative pronoun. Interrogative pronouns are pronouns that are used to introduce a question. The antecedents of interrogative pronouns are unknown because it is what is being asked in the sentence that is missing.
PRO gress,,PRE conceived, PRE cede, POST mortem(?), POST script, PRE amble ... perhaps ?
