In some programming languages, an identifier is permitted to be a letter followed by any number of letters or digits. If L and D denotes the set of letters and digit respectively. Which of the following expression defines an identifier?

A

$$(L+D)^{*}$$

B

$$(LD)^{*}$$

C

$$L(L+D)^{*}$$

D

$$L(LD)^{*}$$