The tedium was occasionally relieved by a large-scale search-and-destroy operation, but the exhilaration of riding the lead helicopter into a landing zone was usually followed by more of the same hot walking, with the mud sucking at our boots and the sun thudding against our helmets while an invisible enemy shot at us from distant tree lines.
a. the paradox in the excerpt illustrates the idea that
b. danger could alleviate the monotony of war.
c. walking was often necessary after flying.
d. the soldiers could not see their enemies.
e. the heat was more difficult to overcome than the enemy

b. Danger could alleviate the monotony of war

Explanation:

The paragraph shows how war is dangerous because the enemy is always anywhere, close enough to attack, so all soldiers must always be alert because of the imminent danger. This danger removes the boredom of the repetitive and tiring activities of war, because while soldiers have to do the same activity, the imminent danger of the war may or may not remove all the monotony.

force is based upon mass and acceleration.the statement is not always true.

it can be explained from newton's second law of motion.

as per newton's second law of motion the rate of change of momentum is directly proportional to the applied force and takes place along the direction of force.

mathematically it can be written as -

                                              ⇒      

here   k is the proportionality constant whose value is 1.

                                                            [a is the acceleration=dv/dt]

hence we see that force not only depends on mass and acceleration but also on velocity and rate of change of mass. force f will depend on mass and acceleration only when mass is constant.

hence the statement that force is based upon mass and acceleration is false.