The name of the city of st. petersburg was changed following the russian revolution

This is true. After the revolution the name was changed from St. Petersburg to Petrograd, which means Peter's city, and a bit later it was further changed into Leningrad. This was because the soviets wanted to completely secularize themselves and remove all figures that were related to the monarchy. In 1991 it changed its name back to St Petersburg.

4.a depression is the answer