Which of the following accurately describes the tax treatment of foreign corporations doing business in the U$.$S$.?$

a$)$ Foreign corporations are exempt from U$.$S$.$ taxation on their U$.$S$.$ source income.

b$)$ Foreign corporations are subject to U$.$S$.$ taxation on their worldwide income.

c$)$ Foreign corporations are only taxed on income earned from sales of goods in the U$.$S$.$

d$)$ Foreign corporations are taxed at a fixed rate regardless of their income levels.