What factors contributed to the expansion of sea trade during the song dynasty?

Song dynasty lasted for 300 prosperous years. Science and technology was explored. International trading was given heavy importance. Expansion of sea trade to neigboring countries are mainly due to the following reasons:

1. The government in Song Dynasty sent people to neigboring countries to persuade traders there to visit China.
2. Ships from China can be seen sailing all over the Indian ocean, replacing Arab and Indian merchants.
3. Their ships were larger and more advanced than that of other merchants.
4. They offered goods in sale.