Identify the characteristic of gold shown in the table that allows gold to be deposited behind the riffl es, while other material fl ows out of the sluice box as tailings. [1]

The characteristic in question is the density - which is relatively high for Gold (and higher than water).

A high density means that the particles of gold will flow towards the bottom of the container.

It's also mentioned in the text: "the more dense gold to drop out"

Water flows along the east coast and across the western tropical pacific towards south america, while they both are directed towards that way,  el niño does not directly affect the southern flow of water along the coast. 

Since wxyz is 4, halfof that would be 2.