Real mother or step mother

Can we chew this for the day

It's about a woman who abandoned her
husband and her 4 year old son for her
rich boyfriend.

... and they both (woman & boyfriend)
eventually traveled to the U.S.A
Out of frustration & desperation the
husband went and married another woman.
Unfortunately the second woman couldn't
bear a child for the husband.

Despite her inability to bear a child,
the 2nd woman took great care of the
boy & treated him like her own son.

Few years later, the husband died in a
car accident.

The 2nd wife did everything in her power
to educate the boy, she engaged in petty
trade such as pepper, Maggie, salt, palm
oil, onion, tomatoes, etc, in the market
square in order to sponsor the boy's
schooling.

The boy only knew his step mother as his
real mother & there4, he called her Mama.

The real mother in the U.S.A did not for
once ask about the wellbeing of her son
thoughtless of sending a penny for the
boy's upkeep, even after she heard that
her ex-husband (son's father) died in an
accident.

Some years later, this boy grew up into
a very intelligent young man waiting for
his B.A Hons degree in Petroleum Engineering
from the University.

The real mother who also could not bear
another child in the U.S since she eloped
with her boyfriend, realized that she had
made a terrible mistake by abandoning her
own son.

She decided to return home to look for her
son & present to him all her wealth acquired
over the years in the States.

She eventually found her son battling
between life & death in the hospital for
kidney failure.

The Doctors demanded huge amount of money
for the operation & the step mother couldn't
afford it.

The real mother stepped in & paid all the
hospital bills & donated one of her own
kidneys & the son was saved.

Shortly after, the final results were
out the son got a distinction & also got
a five year scholarship to pursue his
Masters and PhD degrees in the U.K

At the presentation of the certificate,
the Vice Chancellor gave the mic to the
son to call his mother to come up stage
and receive the certificate.

The moment the Vice Chancellor made the
statement, the real mother got up and
arranged herself waiting to be called
upon whilst the step mother sat down
and looked at the boy with tears running
down her chick.

Assuming you are the son, who would you
call on up stage?

A) Real Mother

B) Step Mother

Remember, you can only call on one person. 

I need your answers ...