雅思英文问题researchers have still to decide whether smell is one

雅思英文问题
researchers have still to decide whether smell is one sense or two- one responding to odours proper and the other registering odourless chemicals in the air. 后半句 one responding to odours proper and the other registering odourless chemicals in the air proper在句子中做什么成分,registering在句子中做什么成分 和什么意思
而且为什么是动词ing形式