What is the main idea of Long-Term Monitoring of the Hudson River? A. The number of large zebra mussels in the Hudson River has gone down in recent years, but scientists predicted that change and are not surprised by it.  

B. The number of small zebra mussels in the Hudson River has unexpectedly gone down in recent years, so scientists have decided to change the focus of their studies on the Hudson River.  

C. The number of zooplankton in the Hudson River has unexpectedly gone up in recent years, so scientists expect the number of zebra mussels to increase as well.  

D. The number of large zebra mussels in the Hudson River has unexpectedly gone down in recent years, but scientists will continue to study the river to understand the invasion’s changing impact

D. The number of large zebra mussels in the Hudson River has unexpectedly gone down in recent years, but scientists will continue to study the river to understand the invasion’s changing impact.

Explanation:

The main idea of the article Long-Term Monitoring of the Hudson River is that the number of large zebra mussels in the Hudson River has unexpectedly gone down in recent years, but that the scientists will continue to study the river to understand the invasion’s changing impact.

The text tells us about the changes occurring in the Hudson River ecosystem. There was an invasion of large zebra muscles, which caused the population of zooplankton to drop. However, recently, the size and the lifespan of the zebra muscles decreased, which led to the growth of the zooplankton population. The scientists don't have the answers to all of the questions that arise from this phenomenon, so they will continue to study the river to understand it better.