<prompt> Positive G effect ?Red outBlack outIncreased cerebral aerial pressureIncreased cardiac output  <response>: Ans. is b i.e., Black outEffects of positive G:Venous return decreases -- hence, cardiac output decreases.Blood rushes towards lower limbs -- hence increased pressure in lower limb vessels.Blood is drawn away from upper body -- hence decreased cerebral aerial pressure.At very high positive G:Difficulty in blood flow to brain -- results in cerebral ischemia unconsciousness.Moments before unconsciousness, there is gray-out.Everything appears gray - Due to ischemia of cone loss of color vision.Eg: Gray-out serves as warning sign for pilots - Indicates need for slowing down aircraft reduce positiveG.Gray-out followed by black-outBlack-out means total loss of vision. 