1. Mg and Al belongs to 3rd period in a modern periodic table ... because group three element has atleast one stable Isotopes occurs naturally.

2.Two advantages of using vegetative propagation.

 i ) - Seedless plants can be grown by this way .. which lost it's ability to reproduce        it's seed.

 ii ) -  Easy and convinient method for farming.

1. Mg and Al belongs to the 3rd period because they have three main shells 
  ex: Mg = 1s^2  2s^2  2p^6 3s^2

2. we have many advantages.... to name a few
    a) many no.of plants can be grown
    b) we can inherit desired characters into them
    c) parthenogenesis helps in producing seed less fruits