How many stereoisomers are there for tartaric acid

The two structures, $\mathrm{I} \text{and} \mathrm{II}$, represent:

(I) 

(II) 

In the given conformation, ${\mathrm{C}}_2$ is rotated about ${\mathrm{C}}_2-{\mathrm{C}}_3$ bond anticlockwise by an angle of $120°$. Then, the conformation obtained is:

The pair of the structures given below represent:

In the following structures, which two forms are the staggered conformation of ethane?

(1) 

(2) 

(3) 

(4)