Answer:
At B part.
Step-by-step explanation:
At B part the nightfall will happen first because the B part is in the shade and will not present in front of the sun. When the earth rotates its own axis, the shaded part which is B comes in front of sun which means day time is started while on the other hand, the A part is in the shade and will experience nightfall so we can say that the nightfall occurs at B part of the earth.