Answer:
Option C, because fossils may be classified into distinct taxa due to the persistence of gaps
Option D, because insufficient numbers of fossil specimens are available
Step-by-step explanation:
In the classification system, the following issues lead to gap -
a) The species that are able to fill the gap with in a taxonomic classification are not identified yet
b) In case these species have been identified they may not have been placed at a proper position due to the missing links
Recently the gap between the dinosaurs and the Archaeopteryx has been filled when it has been finally concluded that in between these two species lies the species that have avian adaptations and have emerged step by step over 60 Myr
Hence, option C and D are correct