Which sentence correctly uses a dash for punctuation?

Responses
A Things have changed a lot—since last school year mainly for the better. Things have changed a lot—since last school year mainly for the better.
B Things have changed a lot since last school year mainly—for the better. Things have changed a lot since last school year mainly—for the better.
C Things have changed a lot since last school year—mainly for the better. Things have changed a lot since last school year—mainly for the better.
D Things have changed—a lot since last school year mainly for the better

C Things have changed a lot since last school year—mainly for the better.

This sentence correctly uses a dash to set off an emphatic statement providing additional information about the main clause. The dash here emphasizes that the changes that have taken place since last school year are primarily positive.