How does pregnancy begin? a) a placenta forms in the uterus b) a sperm reaches an egg in the fallopian tubec) the blastocyst differentiates into two cell types d) contractions begin in the uterine wallapex

I believe it's A.

Explanation: Usually would start with a fertilized egg implanting itself into the walls of the uterus.. Just in case I'm wrong, I'm sorry.

A

Explanation:

sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.

as in mitosis, dna replication occurs during interphase.   in meiosis, it occurs specifically in s phase of interphase 1.