[单选题]
A car makes a U-turn on a flatvv:fn *.3ao:; assthg hori 2z1gil))d:7c/jn*b nobch nv zontal road at a speed of $9\,ms^{−1}$ as shown in the diagram.
The inner radius of the U-turn is $4\,m$ and its outer radius is $10\,m$.
What is the best estimate for minimum acceleration of the car when making the U-turn?
A. $0.9\,ms^{-1}$
B. $2.2\,ms^{-1}$
C. $8.1\,ms^{-1}$
D. $20.2\,ms^{-1}$
