Why was Mendel's work not accepted during his time?

Substances known as 'heredity carriers' are

Some pure black mice were mistakenly mixed with hybrid black mice. The quickest way to discover whether one individual black mouse is pure is:

The modified allele could be responsible for the production of which of the following?

Given below are two statements:
Statement $I$:
Mendel studied seven pairs of contrasting traits in pea plants and proposed the Laws of Inheritance.
Statement $II$:
Seven characters examined by Mendel in his experiment on pea plants were seed shape and colour,flower colour,pod shape and colour,flower position and stem height.
In the light of the above statements,choose the correct answer from the options given below:

How many pairs of true-breeding pea plant varieties were selected by Mendel,which were identical except for one character with contrasting traits?