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In gaseous reactions important for the understanding of the upper atmosphere H2O and O react bimolecularly to form two OH radicals. $∆$H for this reaction is 72kJ at 500 K and Ea is 77 kJ mol-1, then Ea for the bimolecular recombination of two OH radicals to form H2O and O is:

(a) 3 kJ mol-1

(b) 4 kJ mol-1

(c) 5 kJ mol-1

(d) 7 kJ mol-1