[Shifting to hypercritical mode to try to discern the heartburn sources:]

1.) The snag seems to be converting a multiple (theories) into a singular thing (approach); not wrong but arguably awkward.

2.) 'Achievable' carries an expectation of regard to some sort of action; to characterize Europe as achievable sounds a bit non-sequitur.

3.) For a study to "give...statements" smacks of being too direct a relation, such as by cause and effect. An improvement could be for a study to 'form the basis of theories' or 'illustrate trends'.

4.) Again, not really wrong, but knowledge being like 'stuff' makes the 'a' a bit off. Possibly better would be 'thorough knowledge' sans 'a'; Fal's suggestion was even better.

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