Explanation:
A wavefront is the long edge that moves, for example, the crest or the trough. Each point on the wavefront emits a semicircular wave that moves at the propagation speed v. These are drawn at a time t later, so that they have moved a distance s = vt.
The answer is Of frequencies
The most expensive military aircraft is B-2 Spirit bomber, which costs 2.2 billion dollars! The Northrop B-2 Spirit, is also known as the Stealth Bomber.
Source: https://www.aviationcv.com/aviation-blog/2016/top-5-most-expensive-military-aircraft