PLEASE HELP IS THIS A SOCIAL OR CULTURUAL IMPACT OR BOTH??

The Mongol conquests helped to transmit the fleas that carried the bubonic plague, termed the Black Death, from southern China to Central Asia, and from there to Southeast Asia and Europe. It followed familiar paths of trade and military conquest.

Which of these is a disadvantage of hydrogen fuel cells? a. engines that use hydrogen fuel are more efficient than gasoline-powered engines. b. the only emission produced by hydrogen fuel cells is water. c. hydrogen fuel is continually available and useful for heating. d. hydrogen fuel is often purified in a process that emits lots of greenhouse gases.