Many descendants are very different than the things they descended from.
However, just because our knowledge is incomplete does not automatically mean that the expert's argument is flawed.
Not all knowledge of dromeosaur fossils has to relate to the of such fossils.
Here, the assumption being made is that since we have not found any dromeosaur fossils older than the oldest bird fossils, dromeosaurs could not preceded the oldest birds.
What this argument fails to consider is the possibility that there are older dromeosaur fossils that have yet to be found or that have been destroyed.
It states that the problem with this stimulus is its reasoning because no evidence has been found, the proposed hypothesis must be false.
Answer C is discredited because "even if the knowledge of bird and dromeosaur fossils is complete, there could still have been dromeosaurs older than the oldest birds."But this same reasoning could be used to attack answer D ("Known fossils indicate the relative dates of origins of birds and dromeosaurs.") There could be unknown fossils because the knowledge of dromeosaur fossils and bird fossils is not yet complete, and therefore the dinosaur expert's conclusion would be flawed. The best way to prove an Assumption is by using the Assumption Negation technique: logically negate the two contenders and see which of the two logical opposites weakens the expert's conclusion the most: C.
It does not tell how old anything is in years, however, just what order it appeared.
Absolute age, dated in years could only be determined accurately when radiological dating was developed. Both of them will tell you the order of events in earth history, one tells only the order, the other gives the dates by which you can tell the order.
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co.
In geology, relative age was based on a very simple idea; where there is a set of sedimentary layers, the oldest one is on the bottom and the youngest is on the top - so each layer is younger than the one below it and older than the one on top of it. Using this law, geologists can tell which animals appeared first (the ones in the lower layers), and the whole succession of animal life as it developed and the same for anything else the rocks show.
I chose C, and I still think it's the better answer. Knowledge of dromeosaur fossils and the earliest bird fossils is complete.