设 $\varphi(u)$ 为连续的奇函数，则必有
A. $\int_{-a}^a \varphi\left(x^2\right) \mathrm{d} x=0$
B. $\int_{-a}^a \varphi\left(x^3\right) \mathrm{d} x=0$
C. $\int_{-a}^a \varphi\left(x^4\right) \mathrm{d} x=0$
D. $\int_{-a}^a \varphi\left(x^6\right) \mathrm{d} x=0$