60 foot tall OR 60 feet tall

[Freeguy]

This __________ rubber duck is floating in the harbor of Kaohsiung, Taiwan.
a. 60 feet
b. 60 feet tall
c. 60 foot tall
d. tall 60 feet

This test came from one picture taken in Taiwan. A tall duck floating on the sea. Why number c is the correct one and we cannot choose b? Till now, I've heard the structure : 60 feet tall NOT FOOT.

thanks

 

[SoothingDave]

When we use a measurement as an adjective, we use the singular. The duck is 60 feet tall. It is a 60 foot tall duck.

This sub is six inches long. It is a six inch sub.

He weighs 200 pounds. He is a 200 pound man.