Self-study
Generally speaking, if the same crop is sown in a field for several successive years, growth in the later years is poorer than growth in the earlier years, since nitrogen in the soil becomes depleted. ████ ██████ ███████ ██ █ ███████████████ █████ ███ ████ █████████ ███ ██████ ██ ████████ ██ ███ █████ █████████████ ██ ████ ██ ███████ ██ ███ ████ █████ ████ █████ █████ █████ ████ ████ ██ ███ █████ █████ ████ ██ ████ ██ ███ ███████ ██████
"Surprising" Phenomenon
Alfalfa increases the nitrogen in the soil, yet it struggles when planted in the same field year after year.
Objective
A hypothesis explaining this similarity must imply that alfalfa will struggle when planted in successive years. This may be a consequence of increased nitrogen in the soil, or it may be unrelated to nitrogen.
8.
Which one of the following, ██ █████ ████ █████ ██ ███████ ███ ██████████ █████████ █████ ███████ ███████ ███ ███████████████████ ███████
a
Some kinds of ██████ ████ ████ ███████ ███ ███ ████ ██████████ ████ ████ ███ █████ █████ █████ █████ ██ ██████ ██████ ████ █████ █████ ████ █████ ██████ ██ █████ ███ █████
It is not stated whether alfalfa is one such plant, nor is it stated whether alfalfa is typically mixed with other crops. This does not address the success of crops that are planted in the same field in successive years.
b
Alfalfa increases the ██████ ██ ████████ ██ ███ ████ ██ ██████ ████████ ████ ███ ███ ███ █████████ ██ ██ █ ████ ████ ██ ██████ ██ ████ █████ ██ ███████
This simply describes the mechanism by which alfalfa increases nitrogen in the soil. It does not explain why alfalfa struggles when planted in successive years.
c
Certain types of ███████ █████████ ████████ ███████ ██████████ ████ ██████████ ██ ███ ████ ███ ████ ███ █████ ██ ███ ██████ ████ ███████ █████ ███████████
This explains why alfalfa struggles when planted in successive years. Though the soil accumulates nitrogen, it also accumulates toxic substances that are detrimental to alfalfa’s health.
d
Alfalfa increases nitrogen ██ ███ ████ ██ █████ ██ █████ ████ ██ █ ███████ ████ ██ ████ ████████ ██ ███████ ██ ███ █████
This does not state whether the required bacteria exists in alfalfa fields. It establishes a condition necessary for alfalfa to be nitrogen-fixing but takes no position on whether that condition is satisfied.
e
Alfalfa is very █████████ ██ ████████ █ ████████ ████ ██ ██████ ████ ███ ██████ ██ █████ ██████ ██████
This information is useless without more context. No given information suggests that black walnut leaves will more likely be present in fields where alfalfa is grown in successive years.