Tools and hand bones excavated from the Swartkrans cave complex in South Africa suggest thata close relative of early humans known as Australopithecus robustus may have made and usedprimitive tools long before the species became extinct I million Line years ago. It may even havemade and used primitive tools long before humanity's direct ancestor, Homo habilis, or "handy man,"began doing so. Homo habilis and its successor, Homo erectus, coexisted with Australopithecusrobustus on the plains of South Africa for more than a million years.The Swartkrans cave in South Africa has been under excavation since the 1940's. The earliestfossil-containing layers of sedimentary rock in the cave date from about 1.9 million years ago andcontain "EXTENSIVE" remains of animals, primitive tools, and two or more species of apelikehominids. The key recent discovery involved bones from the hand of Australopithecus robustus, thefirst time such bones have been found.The most important feature of the Australopithecus robustus hand was the pollical distal thumbtip, the last bone in the thumb. The bone had an attachment point for a "uniquely human" muscle, thepollicis longus, that had previously been found only in more recent ancestors. That muscle gaveAustralopithecus robustut an opposable thumb, a feature that would allow them to grip objects,including tools. The researchers also found primitive bone and stone implements, especially diggingtools, in the same layers of sediments.Australopithecus robustus were more heavily built- more "robust" in anthropological terms-thantheir successors. They had broad faces, heavy jaws, and massive crushing and grinding teeth that wereused for eating hard fruits, seeds, and underground plant parts. They walked upright, which wouldhave allowed them to carry and use tools. Most experts had previously believed that Homo habiliswere able to "SUPPLANT" Australopithecus robustus because the former's ability to use tools gave"THEM" an innate superiority. The discovery that Australopithecus robustus also used tools meansthat researchers will have to seek other explanations for their extinction. Perhaps their "RELIANCEON" naturally occurring plants led to their downfall as the climate became drier and cooler, orperhaps Homo habilis, with their bigger brains, were simply able tomake more sophisticated toolsWhat does the author suggest is unclear about Australopithecus robustus?a. Whether they used toolsb. What they most likely atec. Whether they are closely related to humansd. Why they became extinct