Question

.

Answer 1

Did you try looking up the experiment?

Answer 2

I'd go for the first one too

Answer 3

you mean how Einstien must have gotten the idea that speed of light is a universal constant in all frames of reference.. well i don't remember where i read it.. but i guess in layman's version it can be thought of this way

we all know for quite a while now (since the time of gallelio) that motion (uniform) is relative concept.. there is nothing like absolute rest or absolute motion.. so now imagine, you are blistering through space at almost speed of light, and u have a mirror in front of u, if ur speed is very close or comparable to speed of light, you won't be able to see ur reflection properly.. cause light can't barely escape ahead of you to go and hit the mirror.. (more like a sonic boom)
and so if i see no reflection i ll say " AHHAAA.. no reflection, thus I AM IN MOTION".. but that is NOT POSSIBLE, cause no relative motion.. HENCE.. even when u are moving, the speed of light MUST be a constant with respect to you.. :D.. pretty neat huh?!