Question

as proved by the transatlantic flights of lindbergh and earhart, postwar airplanes were engineered to fly
for greater distances
with heavier loads
at lower altitudes
at higher altitudes

Explanation:

Lindbergh's and Earhart's trans - Atlantic flights demonstrated the engineering feat of flying long - distance. These flights were across the Atlantic Ocean, which is a very long distance. There is no indication in the context of the trans - Atlantic flights that the focus was on heavier loads (the main challenge was distance, not necessarily payload weight for these record - breaking flights), lower altitudes (they flew at appropriate altitudes for long - distance flight, not particularly lower ones), or higher altitudes (again, not the main aspect highlighted by trans - Atlantic flights). The key achievement was covering the vast trans - Atlantic distance.

Answer:

A. For greater distances