Each statement on it's own is insufficient, as it does not provide sufficient info about all 4 variables.

Both Statements together:

a < c
b < d (You can flip an inequality like this)

When combining 2 inequalities make sure that:

1. You have oriented them such that they have the same symbol ( < , >, etc)
2. You are adding them. DO NOT subtract.

Adding these two inequalities gives us :

a + b < c + d

This is a third inequality. But to get it to answer our prompt, we need to manipulate it a bit more. Technically you can keep adding inequalities to eachother as long as you follow those 2 rules.

Thus we could add our inequality b < d to the above again, to get to our prompt.

a + 2b < c + 2d

For a quick revision of the basic rules of dealing with inequalities, refer to this link:

https://www.platinumgmat.com/gmat_study_ ... equalities

is a+2b<c+2d
a-c<2*(d-b)

(1) a<c, a-c<0 it means that left part of the inequality is - ve but no info about (d-b)
so 1 st insufficient
(2) here (d-b)>0 but no about (a-c) insufficient

together
(a-c)<0 and (d-b)>0, -ve number is always less that +ve so the answer must be C (to me)