The boundary between the crust and the mantle is marked by an abrupt change in seismic velocity called the

Answer collision and mountain formation;

The correct answer is a)  a resource with a limited supply; it will run out.

The us is first  the  u.s. economy  remains the  largest in the world  in terms of nominal  gdp. however, the  u.s. economy  loses its spot as the number one  economy  to china when  measured  in terms of  gdp  based on ppp. in these terms, china's  gdp  is $23.19 trillion exceeds the  u.s. gdp  of $19.42 trillion