Two trains each having speed $30 km/hr$ are leading towards each other on the same straight track. A bird can fly at $60 km/hr$ flies off one train when they are $60 km$ apart and heads directly for the other train. On reaching other train it files directly back to first and so on. How many trips can the bird fly from one train to another before the trains crash?

Question

$$(a)\;10 \quad (b)\;2 \quad (c)\;5  \quad(d)\;infinite$$