The Chesapeake Bay, which is along the coast of the Eastern U. S., is the largest estuary in the country. Many rivers, including the Potomac, empty into this section of the Atlantic Ocean.

During the 1970s, areas of the bay were found to have low levels of oxygen and could not support life. What was the cause of this problem?
A.
A non-native species of fish was introduced into the bay and killed all of the native species.
B.
People removed too many fish from the bay by over-fishing, and the ecosystem never recovered.
C.
High levels of nitrogen and phosphorus from agricultural runoff, sewage, and industrial pollution were flowing into the bay.
D.
The rivers stopped flowing into the bay, and it caused it to dry up and kill the fish.

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.