Which statement best defines the catch-up effect?

a.It is the idea that saving will always "catch up" with investment spending.

b.It is the idea that it is easier for a country to grow fast and "catch up" with richer countries if it starts out relatively poor.

c.It is the idea that rich countries aid relatively poor countries so as to help them "catch up."

d.It is the idea that if investment spending is low, increased saving will help investment to "catch up."