Definition: In this task, you're given passages that contain mentions of a time duration related quesry and we are supposed to write answer to a question that involves event “frequency", which refers to how often an event is likely to be repeated. For example, "taking showers" typically occurs ~5 times a week, "going to saturday market" usually happens every few weeks/months, etc. 
 Note that a lot of the questions could have more than one correct answers. We only need a single most-likely answer. Please try to keep your "answer" as simple as possible. Concise and simple "answer" is preferred over those complex and verbose ones.

Positive Example 1 -
Input: Sentence: Namibia conducts a census every ten years. After independence the first Population and Housing Census was carried out in 1991, further rounds followed in 2001 and 2011. The data collection method is to count every person resident in Namibia on the census reference night, wherever they happen to be. This is called the de facto method. For enumeration purposes the country is demarcated into 4,042 enumeration areas. These areas do not overlap with constituency boundaries to get reliable data for election purposes as well. 
Question: How often does Namibia conduct a census?
Output: every ten years

Positive Example 2 -
Input: Sentence: Plumage is regularly moulted; the standard plumage of a bird that has moulted after breeding is known as the "non-breeding" plumage, or—in the Humphrey-Parkes terminology—"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey-Parkes system as "alternate" plumages. Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines, flight feathers are replaced one at a time with the innermost primary being the first. When the fifth of sixth primary is replaced, the outermost tertiaries begin to drop. After the innermost tertiaries are moulted, the secondaries starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary coverts are moulted in synchrony with the primary that they overlap. A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless. As a general rule, the tail feathers are moulted and replaced starting with the innermost pair. Centripetal moults of tail feathers are however seen in the Phasianidae. The centrifugal moult is modified in the tail feathers of woodpeckers and treecreepers, in that it begins with the second innermost pair of feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail. The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward. Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. The skin there is well supplied with blood vessels and helps the bird in incubation. 
Question: How often does moulting occur in most species?
Output: annual

Negative Example 1 -
Input: Sentence: Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this. The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland; these secretions protect the feathers' flexibility and act as an antimicrobial agent, inhibiting the growth of feather-degrading bacteria. This may be supplemented with the secretions of formic acid from ants, which birds receive through a behaviour known as anting, to remove feather parasites. 
Question: How often do birds groom their feathers?
Output: monthly

Now complete the following example -
Input: Sentence: Despite their close genetic relationship and the ability to inter-breed, there are a number of diagnostic features to distinguish the gray wolves from domestic dogs. Domesticated dogs are clearly distinguishable from wolves by starch gel electrophoresis of red blood cell acid phosphatase. The tympanic bullae are large, convex and almost spherical in gray wolves, while the bullae of dogs are smaller, compressed and slightly crumpled. Compared to equally sized wolves, dogs tend to have 20% smaller skulls and 30% smaller brains.:35 The teeth of gray wolves are also proportionately larger than those of dogs; the premolars and molars of wolves are much less crowded and have more complex cusp patterns. Wolves do not have dewclaws on their back legs, unless there has been admixture with dogs. Dogs lack a functioning pre-caudal gland, and most enter estrus twice yearly, unlike gray wolves which only do so once a year. Dogs require fewer calories to function than wolves. The dog's limp ears may be the result of atrophy of the jaw muscles. The skin of domestic dogs tends to be thicker than that of wolves, with some Inuit tribes favoring the former for use as clothing due to its greater resistance to wear and tear in harsh weather. 
Question: How often do female wolves enter estrus?
Output: 


Answer: once a year.