On oceania's low islands, what is typically the only source of freshwater?

On Oceania's low islands, Rain is typically the only source of freshwater. Oceania includes all of Micronesia, Fiji, and all of Polynesia except the New Zealand. Oceania is often used to describe the regions encompassing the Australian continent and various islands in the Pacific Ocean that are not included in the seven-continent model. 

Ithink the answer is b
not sure