设时间t后的时间为T
甲:X1=0.5a1t²+a1tT+0.5a2T²
V1=a1t+a2T
乙：X2=0.5a2t²+a2tT+0.5a1T²
V2=a2t+a1T
当V1=V2时得（a1-a2）T=（a1-a2）t
因为a1>a2
∴T=t
故：X1=1.5a1t²+0.5a2t²
X2=1.5a2t²+0.5a1t²
X1-X2 =a1t²-a2t²=（a1-a2）t²
∵(a1>a2) ∴a1-a2>0
综上所述得：X1-X2>0
即:X1>X2

选 B