How many pairs of contrasting characters in pea plants were chosen by Mendel?

State the reasons for Mendel's success.

How are the seven traits selected by Mendel distributed across the four chromosomes?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In an experiment, Mendel observed that the $F1$ progeny plants are all tall and none are dwarf.
Reason $R$: Stem height is a contrasting trait, with tall being dominant and dwarf being recessive.
In the light of the above statements, choose the most appropriate answer from the options given below:

Different forms of genes that represent the same trait are called ....

Define / Explain: $True \ breeding \ line$.