Answer the following question given this paragraph: 

that the weak interaction could change quark flavor. It should also change neutrino flavor—that is, any type of neutrino could change spontaneously into any other, a process called neutrino oscillations. However, this can occur only if neutrinos have a mass. Why? Crudely, because if neutrinos are massless, they must travel at the speed of light and time will not pass for them, so that they cannot change without an interaction. In 1999, results began to be published containing convincing evidence that neutrino oscillations do occur. Using the Super-Kamiokande detector in Japan, the oscillations have been observed and are being verified and further explored at present at the same facility and others. Neutrino oscillations may also explain the low number of observed solar neutrinos. Detectors for observing solar neutrinos are specifically designed to detect electron neutrinos ν e produced in huge numbers by fusion in the Sun. A large fraction of electron neutrinos.


Q: A spontaneous change process called neutrino oscillations can occur only if neutrinos have what property?

 Choices:

- velocity

- temperature

- color

- mass

A:
mass
