Walther Bothe and Wilhelm Becker Discovered a New Kind of Particle

Walther Bothe and Wilhelm Becker discovered a new kind of particle radiation in 1930. The particle was later determined to be a neutron. Bothe and Becker generated neutron radiation by

A) heating nuclei to extremely high temperatures to boil the neutrons off.
B) using an isotope that decays by emitting neutrons.
C) using electrons to attract protons away from the nucleus.
D) firing a beam of alpha particles at a beryllium target.