Which of the following fossil sequences is listed from earliest to latest?

a)Orrorin tugenensis, Ardipithecus ramidus, Sahelanthropus tchadensis

b)Ardipithecus ramidus, Sahelanthropus tchadensis, Orrorin tugenensis

c)Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus ramidus

d)Orrorin tugenensis, Sahelanthropus tchadensis, Ardipithecus ramidus