In the final sentence of the third paragraph, the clause "although we do control the conditions of observation functions as a transitional element that
A
explains the role of observation in scientific work
B
suggests that astronomy is less rigorous than experimental sciences
с
refutes a claim regarding the status of astronomy among the sciences
D
suggests that the definition of "experiment has changed over time
E
clarifies the reasons astronomy meets the standards for scientific rigor

Correct. The clause “although we do control the conditions of observation” is a transitional element that establishes a connection between “some sciences, like astronomy” and the authors’ prior definition of scientific observation as, in part, “observation made by qualified observers under controlled conditions” (paragraph 2, sentence 4). This definition emphasizes that scientific observation is more rigorous and intentional than ordinary forms of observation (“more than keeping one’s eyes open”). Therefore, by noting that disciplines like astronomy “control the conditions of observation,” the authors show that these disciplines meet the standards for scientific rigor even though they do not involve experiments.