[单选题]
A distinctive feature o 2/zshd(.ut8ufh2tiv vmu;p4i3pu0s)*vhc 2xpk+p yw nh;a8 m/y 0w df quarks is that they
A. have zero rest mass.
B. have zero charge.
C. have fractional electric charge.
D. are always observed singly, since they do not readily interact with other particles.
