Harappan civilization forms an important landmark in the prehistory of the Indian subcontinent. The civilization offers an excellent example to the modern world in various ways. Their expertise in town planning, water management and harvesting systems as well as drainage mechanism is unparalleled.
I believe the correct answer is: The theory of universal
gravitation.
The famous anecdote of an apple falling from a tree and
striking the head of the young Sir Isaac Newton which stroke the great idea,
helped him come up with The theory of universal gravitation, as a concept and
ever present force.
In the second half of the 19th century, humans used all of the following technological advances to modify and adapt to the physical environment except "the telephone," since although this technology was invented in 1876, it wasn't widely implemented for some time.
