sin(cos(x 1))= x 1 cos(sin(x2))= x2

= & gtx 1-x2 = sin(cos(x 1))-cos(sin(x2))= sin(cos(x 1))-sin(π/2-sin(x2))

= 2 cos[cos(x 1)+π/2-sin(x2)]sin[cos(x 1)-π/2+sin(x2)]

4.I = ∫ [0， 1].....+ ∫ [ 1,+∞) .....

Substitution, let t = 1/x, ∫ [1, +∞)...= ∫ [0, 1] t (2-k)/( 1+t? )? Trembling insanity (abbreviation for Delirium Tremens)

I = ∫ [0， 1][x^k+x^(2-k)]/( 1+x？ )? Advanced (short for deluxe)

Comparison, k=2, I =...= π/4, k = 1, I =...= 1/2.

When k = 3/2 and 1/2, I is equal.

So choose C.