$L_a=\{a^k{b}^n{c}^k{d}^n|k,$ninNat$\}$

$L_b=\{a^k{b}^n{c}^n{d}^k|k,$ninNat$\}$

Which of the following languages is context $-$ free $?$ If it is $,$ show it's context free by writing a context free grammar. Which one isn't? This is confusing me because my professor says one of these IS context free, but because they string must have a$\text{'}$s followed by b$\text{'}$s followed by c$\text{'}$s followed by d$\text{'}$s$,$I$\text{'}$m confused how either one of these could be context free.