Which Statements Are True?

Calculus Level 3

Of the four statements below, which are true?

I. The equation $x^4 -3x+ 1 = 0$ has a unique real solution.
II. The equation $\sin x = x$ has a unique real solution.
III. The equation $3x^5 - 20x^3 + 60x +16=0$ has a unique real solution.
IV. The equation $\tan x = x$ has a unique real solution.