A phone charger transforms AC into DC electricity to charge your mobile phone if the charger is 85% efficient. a) What is the energy needed to obtain 1200 J of DC?

The correct answer is  "c".  it is "c" because melting is an endothermic change, and endothermic get's energy from surroundings. 

The anwser is: wave a has a greater pitch and frequency.

Gotta have a picture ^^

F  you double the  force, you double the acceleration, but  if  you double the  mass, you velocity. all bodies accelerate at the  same  rate regardless of size or  mass.