BOD (Biochemical Oxygen Demand) is a measure of the amount of oxygen consumed by microorganisms in the breakdown of organic matter.
A BOD value of 0-5 indicates very clean water, as there is little organic matter to decompose.
A BOD of 6-15 indicates moderately polluted water.
A BOD higher than 15 suggests highly polluted water.
Since the BOD is 4, it indicates very clean water.
Thus, the correct answer is: (3) Very clean water.


For the circuit shown above, the equivalent gate is:
Let \( f : \mathbb{R} \to \mathbb{R} \) be a twice differentiable function such that \[ (\sin x \cos y)(f(2x + 2y) - f(2x - 2y)) = (\cos x \sin y)(f(2x + 2y) + f(2x - 2y)), \] for all \( x, y \in \mathbb{R}. \)
If \( f'(0) = \frac{1}{2} \), then the value of \( 24f''\left( \frac{5\pi}{3} \right) \) is:
The following are the major sources of Water Pollution: