<P> The Apennines were created in the Apennine orogeny beginning in the early Neogene (about 20 mya, the middle Miocene) and continuing today . Geographically they are partially or appear to be continuous with the Alpine system . Prior to the explosion of data on the topic from about the year 2000 many authors took the approach that the Apennines had the same origin as the Alps . Even today some authors use the term Alpine - Apennine system . They are not, however, the same system and did not have the same origin . The Alps were millions of years old before the Apennines rose from the sea . </P> <P> Both the Alps and the Apennines contain primarily sedimentary rock resulting from sedimentation of the ancient Tethys Sea in the Mesozoic . The northward movement of the African Plate and its collision with the European Plate then caused the Alpine Orogeny, beginning in the late Mesozoic . The band of mountains created extends from Spain to Turkey in a roughly east - west direction and includes the Alps . The Apennines are much younger, extend from northwest to southeast, and are not a displacement of the Alpine chain . </P> <P> The key evidence of the difference is the geologic behavior of the Po Valley in northern Italy . Compressional forces have been acting from north to south in the Alps and from south to north in the Apennines, but instead of being squeezed into mountains the valley has been subsiding at 1 to 4 mm (0.16 in) per year since about 25 mya, before the Apennines existed . It is now known to be not an erosional feature but is a filled portion of the Adriatic Trench, called the Adriatic foredeep after its function as a subduction zone was discovered . The Alps and the Apennines were always separated by this trench and were never part of the same system . </P> <P> The Apennine orogeny is a type of mountain - building in which two simpler types are combined in an apparently paradoxical configuration . Sometimes this is referred to as "syn - orogenic extension", but the term implies that the two processes occur simultaneously during time . Some scientists imagine that this is relatively rare but not unique in mountain building, whereas others imagine that this is fairly common in all mountain belts . </P>

Which letter correctly identifies the location of the apennines