The general solution of \(\frac{{{{\rm{d}}^2}{\rm{y}}}}{{{\rm{d}}{{\rm{x}}^2}}} - 1 = 0\) is

• A. \({\rm{y}} = {{\rm{c}}_1}{{\rm{e}}^{\rm{x}}} + {{\rm{c}}_2}{{\rm{e}}^{ - {\rm{x}}}}\)
• B. \({\rm{y}} = {{\rm{c}}_1}\cos {\rm{x}} + {{\rm{c}}_2}\sin {\rm{x}}\)
• C. \({\rm{y}} = {{\rm{c}}_1}{\rm{x}} + {{\rm{c}}_2} + \frac{{{{\rm{x}}^2}}}{2}\)
• D. \({\rm{y}} = {{\rm{c}}_1}{{\rm{e}}^{\rm{x}}} + {{\rm{c}}_2}\cos {\rm{x}}\)