The reef building corals are not found below 60 m (200 feet) due to:

The reef building corals are not found bellow 60 meters, or better said, they live in shallow waters, because of few reasons. One of the most important are the higher level of oxygen in the shallow parts, they sunlight and the temperature of the water because they need warmer waters.

The investiture controversy began as a power struggle between pope gregory vii (1072–85) and henry iv, holy roman emperor (1056–1106).[2] a brief but significant struggle over investiture also occurred between henry i of england and pope paschal ii in the years 1103 to 1107, and the issue played a minor role in the struggles between church and state in france, as well.by undercutting the imperial power established by the salian emperors, the controversy led to nearly 50 years of civil war in germany, and the triumph of the great dukes and abbots. imperial power was finally re-established under the hohenstaufen dynasty. historian norman cantor:

The answer is c) people evacuated but almost all returned