OG详解-OG6 数学2 Q3

Choice B is correct. If a house from the street is selected at random, the probability of selecting a house that is blue is equal to the number of houses on  the street that are blue divided by the total number of houses on the street. Since there are 2 blue houses on a street with 7 total houses, the probability of selecting a house that is blue from this street is 2​7​.
Choice A is incorrect. This is the probability of selecting a house that is blue from a street on which 1 of the 7 houses is blue. Choice C is incorrect. This is the probability of selecting a house that is not blue from this street. Choice D is incorrect. This is the probability of selecting a house that is blue from a street on which all the houses are blue.