What allows substances to pass back and forth between the placenta and the embryo?

the fetus.

Explanation: The fetus is connected to the placenta by a tube called the umbilical cord. The cord contains two arteries and a vein. Substances pass back and forth between the mother's and fetus's blood through the placenta and cord.

between the mother's and fetus blood through the placenta and cord

9.9 years

a = p(1 + rt) . . account balance after time t at rate r starting with principal p

3000 = 2500(1 + 0.0202t) . . filling in the given numbers

1.2 = 1 + 0.0202t . . divide by 2500

0.2 = 0.0202t . . . subtract 1

0.2/0.0202 = t ≈ 9.901

it will take about 9.9 years for the account balance to reach $3000.

the answer is a) a shiprock was formed by igneous process