53 The function $f(x)=\sin x+\sin 2 x+\sin 3 x$ has
A. a maximum value of 3
B. a maximum value no less than $\frac{3+\sqrt{3}}{2}$
C. a maximum value of $\sqrt{3}$
D. a maximum value of 1
53 B. Since $f\left(\frac{\pi}{4}\right)=\frac{\sqrt{2}}{2}+1+\frac{\sqrt{2}}{2}=1+\sqrt{2}>\frac{3+\sqrt{3}}{2}>\sqrt{3}>1$, therefore C and D do not hold, and A is obviously not true.