Why did student protests at Columbia University in 1968 become violent?

Why did student protests at Columbia University in 1968 become violent?
 
  A. The Black Student Union occupied the campus to protest the draft and end the war in Vietnam.
  B. Civil rights protesters from the Black Student Union and antiwar protesters from the Students for a Democratic Society both occupied the campus at the same time.
  C. The Students for a Democratic Society occupied campus to protest the construction of a gym near Harlem.
  D. The Black Student Union burned draft cards and demanded an end to ROTC programs on campus.