As a vector for the introduction of nonnative species, why is hull fouling particularly difficult to combat?

It is difficult to combat because hull fouling is unintentional

Explanation:

Since hull fouling is unintentional deposits on non-native species into an environment, meaning it is accidental and one may not necessarily know they are introducing a non-native species.  The term comes from when originally non-native species were introduced unintentional, unknowingly and accidentally by attaching themselves to the hulls of ships in one ecosystem where they are native, then traveling and being deposited/inroduced, unintentionally and unknowingly, into the new ecosystem where the species is not native.  Since the non-native species is introduced unintentionally, accidentally and often unknowingly, makes it difficult to detect and control the non-native species, making it particularly difficult and hard to combat and or prevent.

the answer is d. chordata

all matter is made up of atoms

in any element all atoms are the same

elements in a compound are always in a constant ratio: apex