How can an advertiser calculate the view-through rate (VTR) of an AdWords for video campaign?

Q)  How can an advertiser calculate the view-through rate (VTR) of an AdWords for video campaign?

  A)  By dividing the number of clicks by the number of views.
  B)  By looking at the percentage of viewers who watched the video until the last quartile.
  C)  By dividing the number of clicks by the number of impressions.
  D)  By dividing the number of views by the number of impressions.