Which one of the following portfolios cannot lie on the efficient frontier as described by Markowitz?

Portfolio	Expected Return			Standard Deviation
W		9	%		21	%
X		5	%		7	%
Y		15	%		36	%
Z		12	%		15	%

Select one:

a. Only portfolio W cannot lie on the efficient frontier.

b. Only portfolio X cannot lie on the efficient frontier.

c. Only portfolio Y cannot lie on the efficient frontier.

d. Only portfolio Z cannot lie on the efficient frontier.

e. Cannot be determined from the information given.