ACTUAL YIELD VS THEORETICAL YIELD?

This graph gives information on changes in sea ice extent in the arctic ocean over a 30-year span. the overall trend shows in the ice extent. to address the trend, scientists need to ask themselves, one direct consequence of the trend is that

Iknow the answer is a but i don't know why

In which layer of earth do most earthauakes occur a_ inner core b_outer core c_mantle d_crust