For a complex reaction the reaction progress curve
a)has several hills and valleys
b)has only one peak
c)is a flat line
d)shows energy vs. pressure

a) has several hills and valleys

Explanation: Elementary reactions are reactions which takes place in one single step and thus contains only a single peak.

Complex reactions are reactions which cannot take place in one step but takes place in a sequence of number of steps.

In a complex chemical reaction, the reactant molecules pass through different transition states due to bond breaking and bond making. Here the species stays for a very short period, usually called transition period of activated complex.

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< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >