OG详解-OG8 数学2 Q9

Choice D is correct. Since x represents the number of 1-minute segments and y represents the number of 3-minute segments, the total length of the video is 1•x+3•y, or x+3y, minutes. Since the video is 70 minutes long, the equation x+3y= 70 represents this situation.
Choice A is incorrect and may result from conceptual errors. Choice B is incorrect and may result from conceptual errors. Choice C is incorrect and may result from conceptual errors.