The Iowa legislature passed a law banning trucks sixty$-$five feet long or longer from operating on its roads and highways because big trucks do more damage to roads than smaller trucks and are involved in more serious accidents. Prior to the ban, Consolidated Freightways $($CF$)$ had driven big trucks through Iowa on a regular basis, and these trucks were legal in other states. Switching to smaller trucks for trips through Iowa was inconvenient and costly$.$ CF filed a lawsuit contending that the statute was unconstitutional. The United States Supreme Court most likely found that the Iowa statute was:

a$.$ unconstitutional under the privileges and immunities clause.

b$.$ constitutional, because lowa had a compelling state interest in passing the law.

c$.$ unconstitutional under the supremacy clause.

d$.$ unconstitutional under the dormant commerce clause.