The proton’s charge can be taken to have a size of $+1$ unit. With this convention, the charges of the up ($u$) quark, down ($d$) quark and their antiquarks ($\bar{u}$ and $\bar{d}$) are not whole units. Which row in the table gives the correct values for the charges of the $u$, $d$, $\bar{u}$ and $\bar{d}$ quarks?
- A$u : +\frac{2}{3},\ d : -\frac{1}{3},\ \bar{u} : +\frac{2}{3},\ \bar{d} : -\frac{1}{3}$
- B$u : -\frac{2}{3},\ d : +\frac{1}{3},\ \bar{u} : +\frac{2}{3},\ \bar{d} : -\frac{1}{3}$
- C$u : +\frac{2}{3},\ d : -\frac{1}{3},\ \bar{u} : -\frac{2}{3},\ \bar{d} : +\frac{1}{3}$
- D$u : -\frac{2}{3},\ d : +\frac{1}{3},\ \bar{u} : -\frac{2}{3},\ \bar{d} : +\frac{1}{3}$