JNU 2011 pareto optimal alternatives

Posted by vishnu on May 17, 2012; 12:56pm
URL: http://discussion-forum.276.s1.nabble.com/JNU-2011-pareto-optimal-alternatives-tp7563707.html

Suppose there are 3 alternatives x,y and z; and 4 individuals 1,2,3 and 4.
Their ranking of the 3 alternatives are given below
R1:(xy)z
R2:yzx
R3:z(xy)
R4:(xy)z
 
Alternatives inside the brackets are indifferent to each other.If an alternative is written to the left of another alternative, then former is preffered to latter
Then set of pareto optimal alternatives is
a) {x.y}
b) {x.z}
c) {y,z}
d) none of the above.

The answer is c.
Can  any one explain  how the answer is reached?please help...