Judge the following sentence according to the criteria given below: The variable flow of tidal ...

Judge the following sentence according to the criteria given below: The variable flow of tidal currents in two directions is an obstacle to efficient electricity generation by a tidal power plant BECAUSE electrical turbines run best when they run at
 
  a constant velocity. A) The assertion and the reason are both correct, and the reason is valid.
  B) The assertion and the reason are both correct, but the reason is invalid.
  C) The assertion is correct, but the reason is incorrect.
  D) The assertion is incorrect, but the reason is correct.
  E) Both the assertion and the reason are incorrect.