1. 很少发生,罕见：engineering practice 工程实践[惯例] | infrequence 很少发生,罕见 | algicide [化]除海藻的药
2. 很少发生：infrastructure 下部构造 | infrequence 很少发生 | infrequency 很少发生
3. 很少发生; 罕见 (名)：infrastructure 下部构造, 基础, 下部组织 (名) | infrequence 很少发生; 罕见 (名) | infrequency 很少发生; 罕见 (名)
4. 稀少/罕有：infratracheal /气管下的/ | infrequence /稀少/罕有/ | infrequency /稀少/罕有/