C is wrong. An isosceles triangle is an isosceles triangle when the two base angles on the same base are equal.

The title only says that the two bottom angles are equal, which can be understood as different bottoms up and down, and the counterexample is a diamond.

effective digit

It is a number from the first digit on the left that is not 0 to the last digit (including 0, and the scientific counting method does not count the n power of 10), which is called an effective number. Simply put, removing all zeros in front of a number is a valid number.

So d is right, too.