A alone is sufficient.
St2 is just re-iterating the statement in the question.
Can you explain how A is sufficient?
In May, Josep sold 40% of the computers that were sold by the store at which he works. In June, Josep sold 60% of the computers sold by the store. Did Josep sell more than twice as many computers in June as he did in May?
St 1: The number of computers that the other employees sold was higher in June than in May.
Josef sold 40 % of the total computers sold in May. He sold 60% of the total sold in June.
In May other employees sold 60% computers and in Jun they sold 40% of the total.
St 1 : 40% of the total sold by other employees in June is more than 60% of the total sold by the other employees in May.
This means that the total # of computers sold in Jun was more than the total sold in May, coz the 40% of the # in Jun > 60% of the # in May. Hence the # in Jun > # in May.
Now lets look back at Josep. Even if the # of computers sold in Jun and May was same, Josep would have sold 1.5 times the computers he sold in May. Also we know that the total # of computers sold in Jun is greater than in May, which means the # of computers Josep sold in Jun is more than 1.5 times the # he sold in May.
Hope it helps...