An angular acceleration graph of the knee has equal areas above and below the zero

degrees$/$second axis $($the x$-$axis$).$ Which of the following is true? $(1)$

a$.$ The knee has undergone no angular displacement

b$.$ The knee has been moved through its full range of motion

c$.$ The knee has had zero angular acceleration

d$.$ None of the above are correct

e$.$ The change in angular velocity of the knee was zero

f$.$ Both e and b