阅读理解：科技与自然环境类语篇 — 英语填空题
题目
A river-monitoring team installed sensors that measure dissolved oxygen every twenty minutes. The system sends an alert only when two readings in a row are below 5 mg/L. On Tuesday, one reading was 4.6 mg/L, but the next was 5.2 mg/L. The team received no alert because ___.