Why did Becquerel expose uranium crystals to sunlight in his initial experiment?

Below are the choices that can be found elsewhere:

He wanted to show that warmed crystals radiate heat.

He wanted to show that the crystals emit X-rays.

He wanted to show that the crystals emit light after absorbing light.

He wanted to show that the crystals would become radioactive when they were exposed to sunlight.

The answer is " He wanted to show that the crystals emit X-rays."