|Question 4Verbal

Source Texts

Text
Economists have long observed that firms in related industries tend to cluster in the same area, as is the case in Leeds, UK, where many thread and yarn spinning firms as well as textile weaving firms are located. The factors causing such clustering are        what drives firms in certain industries to cluster may not be especially relevant among other industries.
Which choice completes the text with the most logical and precise word or phrase?
unknown
A
impressive
B
focused
C
variable
D