Prestressed concrete is often used in modern construction because:

a. its unique strength allows predetermined engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads

b. it is "scored" in certain areas, which allows it to break apart predictably when a building explodes

c. it doesn't normally have end anchors, which makes it easier to attach to adjacent members

d. both b and c

A.  Its unique strength allows predetermined engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads

Explanation:

Prestressed concrete is a structural material that allows for predetermined, engineering stresses to be placed in members to counteract the stresses that occur when they are subject to loading.

It combines the high strength compressive properties of concrete with the high tensile strength of steel.

‘Building Construction Handbook’ (6th ed.), CHUDLEY, R., GREENO, R., Butterworth-Heinemann (2007)

Then the correct answer is:

A.  Its unique strength allows predetermined engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads

+15-35=-20

going in opposite directions