B

Step 1: Using the inverse square law of light intensity, the brightness (or luminosity) received by a planet is inversely proportional to the square of the distance from the star. For Earth around the Sun, the distance is `r`, and for the other planet, it is `2r`.

Step 2: The intensity of light (I) received by the Earth is given by `I = L / r^2`, where `L` is the luminosity of the sun (a constant). Thus, for the planet:
I_planet = L / (2r)^2 = L / (4r^2) = I_earth / 4.

Step 3: Since the other star is much brighter but has the same mass, and we assume the average life span of a man is directly related to the luminosity received, we can express it as: `L_planet = 4 * I_earth`.

Step 4: If the average lifespan on Earth is 70 years, the lifespan on the planet can be estimated in terms of luminosity received. The equation becomes: Average lifespan_planet = Average lifespan_earth * (I_earth / I_planet) = 70 * 4 = 280 years.

Therefore, the average lifespan of a man on the other planet is approximately 280 years.