For #$18($ exercises you must also provide the resulting language to get full credit.

$18.$ Give examples of languages $L_1$ and $L_2$ over $\{a,b\}$ that satisfy the following descriptions.

a$)L_1$ is regular, $L_2$ is nonregular, and $L_1{L}_2$ is regular.

b$)L_1$ is regular, $L_2$ is nonregular, and $L_1{L}_2$ is nonregular.

c$)L_1$ is regular, $L_2$ is nonregular, and $L_1{L}_2$ is regular.

d$)L_1$ is nonregular, $L_2$ is nonregular, and $L_1{L}_2$ is regular.

e$)L_1$ is nonregular and $L_1^{**}$ is regular.