Answer: 1500 m/sec. ( b )
Solution: 
V= f (λ) 
Speed = frequency. Wavelength 
And frequency = 1/time period 
First we will find the frequency : 
Frequency = 1/time period 
1/0.006 = 166.67 Hz 
 Using this in the equation: 
Speed = frequency. Wavelength 
V= f (λ) 
9 × 166.67 
 = 1500.03 ≈ 1500 
V (Speed) = 1500 m/s. 
 Note: Do not forget to mention the units or otherwise they may cost you marks!