Final answer:
The lowest base level for most streams is true to be sea level, which is the ultimate limit of erosion for a stream. However, a pebble dropped in the water is false to be a pulse wave, as a pulse wave is a single disturbance moving through a medium, which differs from the ripples caused by a pebble.
Step-by-step explanation:
The statement that the lowest base level for most streams is sea level is true. Base level refers to the lowest point to which a stream can erode its channel. For most streams, this lowest point is indeed sea level, which acts as the ultimate base level. However, local base levels can exist, such as a lake, a reservoir, or a harder layer of rock. In the geological past, sea levels have varied significantly, but today's streams continue to cut down towards the current sea level, which is the ultimate limit of stream erosion.
Changes in sea level over geological time have had significant impacts on the landscape and can be evidenced by the study of fossils and sediment layers. But a pebble dropped in water, while related to fluid dynamics, does not represent a pulse wave, which is a single disturbance that moves through a medium from one point to another. Hence the statement "A pebble dropped in water is an example of a pulse wave" is false.