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
3. B For any real number $x$, according to the problem, we have $f(x)=-f(x), f(x+2)=$ $-f(x)$,
so $f(x+4)=-f(x+2)=f(x)$.
Also, $f(x)=x(0 \leqslant x \leqslant 1)$,
so when $-1 \leqslant x \leqslant 0$, $f(x)=x$,
when $1 \leqslant x \leqslant 3$, $-1 \leqslant x-2 \leqslant 1$. Thus, $f(x)=-f(x-2)=$ $-(x-2)$,

In summary: $f(x)$ is a function with a period of $T=4$, its graph is shown in Figure $1-2$, hence $f(7.5)=f(3.5)=$ $f(-0.5)=-0.5$.