Which of the following communication standards requires line-of-site with no obstacles in order to operate properly?

Which of the following communication standards requires line-of-site with no obstacles in
order to operate properly?

A.
Bluetooth

B.
IR

C.
RF

D.
WiFi

Explanation:

http://www.di.unisa.it/~vitsca/RC-0910I/ch04.pdf (page 25)