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If f is a real-valued differentiable fun...

If f is a real-valued differentiable function such that `f(x)f'(x)lt0` for all real x, then -

A

f(x) must be an increasing function

B

f(x) must be a decreasing function

C

`|f(x)|` must be an increasing function

D

`|f(x)|` must be a decreasing function

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