Identify the correct sentence. a. its imperative that the dishes are washed before the guests arrive. b. it's imperative that the dishes are washed before the guests arrive. c. it's imperative that the dishes are washed before the guests' arrive. d. its imperative that the dishes are washed before the guest's arrive.

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.