How to Replace only a word in C# and not the specific letters in a word?

Question

I want to replace some string as using .Replace in C#

i have a string contains words like

this is 1st answer this is 2nd answer this is 3rd answer this is 4th answer

now when i try to replace "is" from the above string

"this" also gets replaced and out comes th 1st answer th 2nd answer th 3rd answer th 4th answer

i only want to replace a specific word and not the half word

Kindly help