Identify the type of organic reaction that would best accomplish each conversion.
$\begin{array}{ll}{\text { a. alkyl halide } \rightarrow \text { alkene }} & {\text { c.alcohol }+\text { carboxylic acid } \rightarrow \text { ester }} \\ {\text { b. alkene } \rightarrow \text { alcohol }} & {\text { d. alkene } \rightarrow \text { alkyl dihalide }}\end{array}$