Some interesting pictures showing the differences of the cars between Bahrain and China:
In order to get sufficient cooling for race at Bahrain almost all teams had to increase the size of hole at the rear, which has a negative effect on the aerodynamics(Coanda effect not as efficient).
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Even Ferrari had open up the rear.
And they even added two intakes:
Only Mercedes were able keep the rear closed, despite having the strongest or 2nd strongest power unit.