Which of the following is a good example of agglomeration?

a$.$ Hundreds of small, medium, and large internet$-$based, high$-$tech companies are located in Silicon Valley, California.

b$.$ A manufacturing plant is hiring highly skilled, extensively trained employees from other manufacturing plants.

c$.$ A firm is setting up a licensing agreement with a foreign firm to learn the foreign firm's secrets$.$

d$.$ Dozens of European companies are pursuing FDI in other European countries.