Karan wants to measure the length of a box using a ruler as shown in the illustration given below. Where should he place his eyes while measuring the length of the box?

•  The correct position of the eye is very important for taking measurements.
• Our eyes must be exactly in front of the point where the measurement is to be taken.
• The reading will be a little different from positions A, C and D and it will be exact only from point B as the eye is perfectly aligned with the edge of the box.

Therefore, Karan should place his eyes at point B while measuring the length of the box.