Explanation

The correct temperature where Celsius and Fahrenheit are equal is -40 degrees. Since both A and B state '40 degree', and the actual answer is -40, there might be a typo in the question or options provided; however, based on standard knowledge, 40 is not the correct numerical value for equality. Option B repeats option A's text but has no distinct reason to be correct over A. Options C and D state '10 degree', which is incorrect as it does not satisfy the conversion formula.