Two motor-cyclists John and Kevin were $800 \text{km}$ apart and travelling towards each other at a constant speed. They started at the same time, meeting after $8$ hours. If Kevin started $1\dfrac{1}{2}$ hours later than John, they would be $70 \text{km}$ apart $8$ hours after John started. At what speed was John travelling in $\text{km/h}$? 
选项为:A:$$50$$  B:$$51 \frac{2}{3}$$  C:$$52 \frac{1}{2}$$  D:$$53 \frac{1}{3}$$  E:None of the above  请回答其中一个选项.