1. 100,000% A. I simply googled C12 and it's the most stable Isotope of Carbon...
2. A as well.. 99% sure.. and so far my 99% sure has been 100% correct so ... :p
Answer:
W = 0
Explanation:
We are given with, a construction worker is carrying a load of 40 kg over his head and is walking at a constant velocity. He travels a distance of 50 m.
The work done by an object is given by :

F = ma
So,

m is mass
a is acceleration
d is displacement
The worker is moving with constant velocity, its acceleration will be 0. So, the work done by the worker is 0.
A )voltage
(i.e differance in electricity field)
Physical properties are 36.00 cm