Final answer:
The reason for Mark coming home to an empty house is that Becky has started going out with friends after school.
Step-by-step explanation:
Mark comes home to an empty house because his sister Becky is busy with afterschool clubs and socializing with friends, leaving him to start dinner by himself.
Mark's sister Becky, who previously shared the responsibility of starting dinner, is now spending her afternoons participating in afterschool clubs and going out with friends, leaving Mark to manage the chore alone. This is a common scenario where family dynamics change as children grow older and become more involved in social activities outside of the home.