Analysis of test questions: the sky at the bottom of Qingxi is a phenomenon of plane mirror imaging, that is, the sky is a scene, and the water surface is imaged by a plane mirror in water, which is caused by the reflection of light.
A, plane mirror imaging is caused by light reflection, so it is the same as the formation principle of the sky at the bottom of Qingxi. Therefore, A is correct.
B, pinhole imaging reflects the straight-line propagation of light, so B is wrong.
C. The decomposition of sunlight is a dispersion phenomenon of light, which is caused by the refraction of light. So, c is wrong.
D, correct vision, whether concave lens or convex lens, all use the refraction of light. So, d is wrong.
So choose a.
Comments: It is the starting point to solve this problem to make it clear that the sky is formed by the reflection of light at the bottom of Qingxi.