1. Observing nature is one of the oldest _ methods in making discoveries ( science ) 
2. most biologist consider a species _ if they expect it would die off completely in less than 20 years ( endanger ) 
3. the search was called off for the sailors who _ in the storm ( appear ) 
4. in the 1830's Charles Darwin _ observed plants and animals in many parts of the world ( care ) 
5. The _ of sound waves can be demonstrated with two speakers connected to the same sources of sound ( interfere )