Static Text update according to GUI List box callback

I am trying to update my static text with tag text105 on my gui according to the value selected from the list box. The code I have developed is below, but it failed to display the value. Kindly spot the error.

function listbox1_Callback(hObject, eventdata, handles)

a=get(handles.listbox1,'value')

if (a==Natural-Gas)

Cv=12500;

set(handles.text105,'string',Cv)

drawnow;

elseif (a==Propane-Butane)

Cv=11950;

set(handles.text105,'string',Cv)

drawnow;

elseif (a==Fuel-Oil)

Cv=9520;

set(handles.text105,'string',Cv)

drawnow;

elseif (a==Diesel)

Cv=1000;

set(handles.text105,'string',Cv)

drawnow;

elseif (a==Brown-Coal)

Cv=3500;

set(handles.text105,'string',Cv)

drawnow;

elseif (a==Woods)

Cv=2500;

set(handles.text105,'string',Cv)

drawnow;

elseif (a==Electricity)

Cv=860;

set(handles.text105,'string',Cv)

drawnow;

end