Answer:
"The actual construction of the pyramids has baffled historians for years."
Between sentences 2 and 3
The correct answer is 1: Illustration-example.
Illustrations and examples are introduced by the transition phrases such as "for instance" and "for example". Some other linking words for giving examples are "to illustrate", "namely", and "such as". These linking words do not necessarily have to be at the beginning of the sentence.
