Randomized control trials are often considered the gold standard in estimating causal effects.

Which of the following conditions make RCTs better than quasi$-$experimental methods?

$1.$ Observing counterfactual

$2.$ Sample size

$3.$ Attrition rate

$4.$ Variance

a$.1,4$

b$.3\mathrm{.}4$

c$.2,3,4$

d $1,3,4$