The composite function g f is a constant function if

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1. Af(x)= ( ⁻¹ x, ⁻¹ x ) and g(x, y)=x+y
2. Bf(x)=[x] and g(x)=x-[x] , where [x] denotes the largest integer less than or equal to x .
3. Cf(x)= x |x| and g(x)= ( 2 x )
4. Df(x)=| x| and g(x)=x-[x] .