Although blue herons can still be found along the Maine coast, their population has declined dramatically when

Although blue herons can still be found along the Maine coast, their population has declined dramatically when compared to the 1970’s, a period in which there were fewer bald eagles competing with them for food and nesting sites.

A)their population has declined dramatically when compared to the 1970s
B) their population has declined dramatically compared with the 1970s
C)their populations have declined dramatically since the 1970s
D) their populations have declined in comparison to those of the 1970s
E) their population has declined in comparison with that of the 1970s

Someone please let me understand whats problem with


OA C

Source: Veritas Prep