问题

Three people got killed by two explosions.—When was __________?—________ was on April 15 __________ the 2013 Boston Marathon was being held.
A:that; It; when
B:this; This; that
C:this; It; when
D:that; It; that
B:this; This; that
C:this; It; when
D:that; It; that
正确答案是: