Consider the following Java method that calculates sum of all elements in an array:

public static int sum$($int$[]$ arr$)\{$

int total $=0$;

for $($int i $=0$; i $<$ arr.length; i$++)\{$

total $+=$ arr$[$i$]$;

$\}$

return total;

$\}$

a$)$ What is the time complexity of the sum method in terms of Big$-$O notation? Justify your answer. $(1$ mark$)$

b$)$ Suppose the array arr has n elements. If the array contains very large numbers, what is the best$-$case time complexity of the sum method? Explain briefly. $(1$ mark$)$