Question

Which sentence is an example of chronological structure in "The City Without Us"?
 
A.
Unheated buildings would result in a destructive freeze-thaw cycle.
 
B.
Without daily pumping, Manhattan's subway tunnels would be flooded.
 
C.
After 20 years, rusted beams in buildings will cause them to start collapsing.
 
D.
If flooding were to go unchecked, steel support beams in buildings would rust.

Answer 1

C. 
After 20 years, rusted beams in buildings will cause them to start collapsing.

Answer 2

the answer is: C. After 20 years, rusted beams in buildings will cause them to start collapsing.

Chronological structure refers to a technique that put events in proper sequence starting on the earliest time to the latest. The phrase 'after 20 years' indicate the author first explain the situation before the 20 years process occurs and continue in proper sequence to explain what happen next.