[单选题]
An object sits on a rottyqx9e d.6kd5ating pz-dlei s78su( u wi,4horizontal disc. The disc rotates from rest with uniform angular acceleration α. After -d zspe8wil7uus4( i,time t the object slides off of the disc.
What is the linear speed of the object at this time?
A. $αt$
B. $rαt$
C. $\frac{1}{2}αt^2$
D. $\frac{1}{2}rαt^2$
