How many significant figures are there in the measurement 64,000 mg? a. 2 b. 3 c. 4 d. 5 (possibly a but idk)

I'm pretty sure that significant figures is just the amount of numbers there is. So, in this case I think the answer would be D. 5

the ozone layer acts as a filter for the shorter wavelength and highly hazardous ultraviolet radiation (uvr) from the sun, protecting life on earth from its potentially harmful effects. when the sky is clear, there is an inverse relationship between stratospheric ozone and solar uvr measured at the earth's surface.