The okinawa and amami great island are the biggest among the ryukyu islands. how were these bigger islands created?

The Okinawa and Amami Great Island are the biggest among the Ryukyu Islands. They were generally volcanic in origin and have mountainous terrain. The Amami Islands are limestone islands of coralline origin.  The Okinawa ans Amami Great Island were created by volcanic activity from the sea bed, the pressure from the volcanic activity managed to get massive amounts of magma up to the surface, thus lifting the sea floor and creating an island. At that time, there were numerous volcanic activities nearby, thus the little islands created later merge into one bigger island. 

The correct answer is b