Two cars P and Q are moving in a racing track continuously for two hours. Assume that no other vehicles are using the track during this time. The expressions relating the distance travelled $d$ (in km) and time $t$ (in hour) for both vehicles are given as

P: $d=60 \: t$

Q: $d=60 \: t^2$

Within the first one hour, the maximum space headway would be

- $15$ km at $30$ minutes
- $15$ km at $15$ minutes
- $30$ km at $30$ minutes
- $30$ km at $15$ minutes