3. (1996 College Entrance Examination Math Question) Let $f(x)$ be an odd function on $(-\infty,+\infty)$, $f(x+2)=-f(x)$, when $0 \leqslant x \leqslant 1$, $f(x)=x$, then $f(7.5)$ equals
A. 0.5
B. -0.5
C. 1.5
D. -1.5