Q: This task involves annotating the answer type to a given question that involve some kind of complex reasoning (including numerical reasoning). Note that the questions require looking at more than one part of the passage to answer. There are 3 possible answer types (i) spans, (ii) numbers and (iii) dates. If the answer can be found in the passage, label it as "span". If the answer is a number, label as "number". Similarly, label "date" if you think the answer to the given question is a date.
Passage: The states with the greatest percentage or professing Catholics are central states, namely Guanajuato (96.4%), Aguascalientes (95.6%) and Jalisco (95.4%), whereas southeastern states have the least percentage of Catholics, namely Chiapas (63.8%), Tabasco (70.4%) and Campeche (71.3%). The percentage of professing Catholics has been decreasing over the last four decades, from over 98% in 1950 to 87.9% in 2000. Average annual growth of Catholic believers from 1990 to 2000 was 1.7% whereas that of Non-Catholics was 3.7%. Given that average annual population increase over the same time period was 1.8%, the percentage of Catholics with respect to total population is still decreasing.
Question: How many years was the time span where the percentage of professing Catholics was decreasing?
A:
number