In March Cost is = $30$, whereas in Jan Cost is = $29$
So, the difference between cost is = $(30-29)= 1$
In May Cost is = $34$ , whereas in March cost is = $30$
So, the difference between cost is = $(34-30)= 4$
In July Cost is = $35$ , whereas in May cost is = $34$
So, the difference between cost is = $(35-34)= 1$
In September Cost is = $37$ , whereas in July cost is = $35$
So, the difference between cost is = $(37-35)= 2$
∴ In May the total increase in the Cost highest as compared to the Cost two months ago