Laplace transform of e-at u(t), is ______, where u(t) is unit step.

• A. \(\frac{1}{{s + a}}\)
• B. \(\frac{1}{s}\)
• C. \(\frac{1}{{s\left( {s + a} \right)}}\)
• D. \(\frac{s}{{s + a\;}}\)