Assertion (A) : Careful analysis of records shows that extinctions across taxa are not random. 
Reason (R) : For many taxonomic groups, species inventories are more complete in temperate than in tropical countries.

A and R both are true but R is not correct explanation of A.

Assertion (A) is true because careful analysis of extinction records often reveals patterns that are not random. Extinctions can be influenced by various factors such as habitat destruction, climate change, overexploitation, and invasive species, among others. These factors can affect certain taxa more severely than others, leading to non-random patterns of extinction.

Reason (R) is also true but it does not directly support Assertion (A). The completeness of species inventories in temperate versus tropical countries may influence the detection of extinctions but does not directly explain why extinctions across taxa are not random. While it's true that more complete inventories may lead to a better understanding of extinction patterns in certain regions, other factors such as differences in environmental pressures, species interactions, and evolutionary histories also play significant roles.