Data Sufficiency

Iam bamboozled with the Ans.

Total distance = 10*1.6 = 16 km
total time = t
is t>2 hrs

st1:
speed less than 6.4km/hr
that means the speed will always be less than 6.4
if the speed is 6.4, then he will cover only 12.8 km in 2hrs
that means even at 6.4 he requires more than 2 hrs to cover 16 km
SUFF

st2:
on avg he took more than 9 min / km
lets say he took 9min/km
total time for 16km = 16*9=144min
144min is more than 120 min (2 hrs)
SUFF

ans D