Which choice most logically completes the text?

Source Texts

Text

There are over 150 species of the cactus genus Mammillaria throughout the Americas, but their survival can be threatened by high precipitation and dense vegetation that blocks sunlight. Researchers have located species from the genus in almost every state in Mexico, with several of them, like M. schumannii, restricted to only one state. The fact that this genus has not been observed in parts of the Sierra Madre Occidental mountain range has been attributed to a lack of appropriate habitat, but much of the landscape in this area is notoriously inaccessible, which suggests that _____

the current methods of collecting and tracking Mammillaria species throughout Mexico may cause an overestimation of the number of species in this genus.

M. schumannii may have been overlooked in parts of the Sierra Madre Occidental mountain range because of its similarity to another species.

the dense vegetation and high annual precipitation levels in parts of the Sierra Madre Occidental mountain range impede the ability of Mammillaria species to survive.

the perceived absence of Mammillaria in parts of the Sierra Madre Occidental mountain range may be due to insufficient exploration of the region.