阅读下列短文，根据短文内容填空。在未给提示词的空白处仅填写1个恰当的单词，在给出提示词的空白处用括号内所给词的正确形式填空。

Why do humans prefer some smells over others? One theory, increasingly (support) by experts, suggests that smell preferences are learned. It's easy to explain how we determinesmells are dangerous or not: we learn. This has been adopted to ensure easier detection of gas leaks. Gas naturally.(have) no recognisable smell. However, a strong smell is added so that we can raise the alarm when we detect the smell associated with danger.