Although the Swedish chemist Svante Arrhenius is often credited with first quantifying the greenhouse effect in 1896, other researchers had laid the groundwork decades earlier. Indeed, by the time Arrhenius published his landmark paper, other scientists—including the American investigator Eunice Newton Foote, whose pioneering experiments in 1856 demonstrated the heat-trapping properties of carbon dioxide—_________ already observed that the gas absorbs infrared radiation.
Which choice completes the text so that it conforms to the conventions of Standard English?
- Ahave
- hadAnswer
- Cwould have
- Dwill have
Answer
had
The passage describes events that occurred in the past, specifically focusing on scientists who made observations before Arrhenius published his paper in 1896. To describe an action that was completed before another past action (represented here by the simple past verb 'published'), the past perfect tense is required. The past perfect tense is formed using the auxiliary verb 'had' combined with the past participle 'observed', making 'had' the correct choice.
Step-by-Step Solution
Key Concept
The past perfect tense is used to describe an action that occurred before another action in the past, often marked by phrases like 'by the time'.